Self-contained fpm

[develar]

I am author of https://github.com/electron-userland/electron-builder and we use fpm to build deb packages.

And I am tired to handle Ruby&gem issues — it is awkward to require Ruby & gems.
I have prepared self-contained fpm — https://github.com/develar/fpm-self-contained/releases (no changes in the existing fpm code, thanks you! — develar@05a97e4).

May be it will be interested for someone else.

Currently, only OS X build — works on my machine and on Travis CI servers.
Unfortunately, travelling-ruby is not maintained — phusion/traveling-ruby#62 (comment) So, it is not easy to prepare self-contained build for Linux because fpm uses native extensions (on OS X you can build extensions on your host machine as part of packaging process, but it is not possible for Linux).

[develar]

One change will be required — get rid of explicit json dependency (see rails/rails#23453)

[develar]

Release for Linux (latest ruby 2.3.1) — https://github.com/develar/fpm-self-contained/releases