Aeolus image rubygem versionfix

Fixes https://github.com/aeolus-incubator/dev-tools/issues/18

I tested this with my latest tim puppet changes and was able to add providers and build mock images.  That is, upstream aeolus-image-rubygem seemed to work fine for me.

For a release in the near future, we may want to a given tag, e.g.

    FACTER_AEOLUS_IMAGE_RUBYGEM_BRANCH=v0.6.0-1