Convert scripts/install-latest.sh to a shim that loads the real script

[Daniel15]

Summary
I moved the installation script to the site, as that's a better place for it. Our documentation has always used https://yarnpkg.com/install.sh as the URL to the script, so everything should keep working as-is. However, some users may have hard-coded a link to the file directly in this repo (eg. https://raw.githubusercontent.com/yarnpkg/yarn/master/scripts/install-latest.sh). To handle this case, I'm converting the script to a small shim script that downloads the real one.

Test plan
Test with no params, ensure "This script has moved to" message appears, and it correctly downloads and executes that script:

daniel@Daniel-PC:/mnt/c/src/yarn/scripts$ ./install-latest.sh
This script has moved to https://yarnpkg.com/install.sh, please update your URL!
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  6074    0  6074    0     0   8911      0 --:--:-- --:--:-- --:--:--  8906
Installing Yarn!
> Downloading tarball...

[1/2]: https://yarnpkg.com/latest.tar.gz --> /tmp/yarn.PTyJ3Qlrs1.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    92  100    92    0     0    758      0 --:--:-- --:--:-- --:--:--   766
  0     0    0   595    0     0   1120      0 --:--:-- --:--:-- --:--:--  1120
100 3530k  100 3530k    0     0  1456k      0  0:00:02  0:00:02 --:--:-- 1998k

[2/2]: https://yarnpkg.com/latest.tar.gz.asc --> /tmp/yarn.PTyJ3Qlrs1.tar.gz.asc
100    96  100    96    0     0   1547      0 --:--:-- --:--:-- --:--:--  1547
  0     0    0   600    0     0   3303      0 --:--:-- --:--:-- --:--:--  3303
100   832  100   832    0     0   2840      0 --:--:-- --:--:-- --:--:--  270k
> Verifying integrity...
gpg: Signature made Mon 28 Nov 2016 03:43:55 PM STD using RSA key ID FD2497F5
gpg: Good signature from "Yarn Packaging <[email protected]>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 72EC F46A 56B4 AD39 C907  BBB7 1646 B01B 86E5 0310
     Subkey fingerprint: 6A01 0C51 6600 6599 AA17  F081 46C2 130D FD24 97F5
> GPG signature looks good
> Extracting to ~/.yarn...

Test with --version param, ensure it's passed to the real script:

daniel@Daniel-PC:/mnt/c/src/yarn/scripts$ ./install-latest.sh --version 0.17.8
This script has moved to https://yarnpkg.com/install.sh, please update your URL!
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  6074    0  6074    0     0  48782      0 --:--:-- --:--:-- --:--:-- 48592
Installing Yarn!
> Downloading tarball...

[1/2]: https://yarnpkg.com/downloads/0.17.8/yarn-v0.17.8.tar.gz --> /tmp/yarn.e5MluuXhYl.tar.gz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100    92  100    92    0     0     82      0  0:00:01  0:00:01 --:--:--    82
  0     0    0   595    0     0    374      0 --:--:--  0:00:01 --:--:--  2945
100 3525k  100 3525k    0     0   986k      0  0:00:03  0:00:03 --:--:-- 2881k

[2/2]: https://yarnpkg.com/downloads/0.17.8/yarn-v0.17.8.tar.gz.asc --> /tmp/yarn.e5MluuXhYl.tar.gz.asc
100    96  100    96    0     0    310      0 --:--:-- --:--:-- --:--:--   310
  0     0    0   600    0     0   1412      0 --:--:-- --:--:-- --:--:--  1412
100   832  100   832    0     0   1485      0 --:--:-- --:--:-- --:--:--  1485
> Verifying integrity...
gpg: Signature made Mon 21 Nov 2016 10:18:40 PM STD using RSA key ID FD2497F5
gpg: Good signature from "Yarn Packaging <[email protected]>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: 72EC F46A 56B4 AD39 C907  BBB7 1646 B01B 86E5 0310
     Subkey fingerprint: 6A01 0C51 6600 6599 AA17  F081 46C2 130D FD24 97F5
> GPG signature looks good
> Extracting to ~/.yarn...

Closes #2058

[bestander]

@Daniel15, please rebase