[fake-s3] Install fake-s3 from a branch on GitHub

Changing this image to build fake-s3 from a branch I have on GitHub
that works around issue #66:

  https://github.com/jubos/fake-s3/issues/66

Once my changes are merged and a new version is pushed to Ruby Gems,
we can revert to just installing with `gem install`.
Merge pull request #5 from cnry/bison/fake-s3-from-git

[fake-s3] Install fake-s3 from a branch on GitHub