Upgrade pyyaml to 4.2b1

[qboot]

Related issue #6441

Addresses https://nvd.nist.gov/vuln/detail/CVE-2017-18342.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "master" [email protected]:qboot/compose.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[shin-]

Thanks!