Fix breaking specs on Travis

I can't figure out the one about .bundle/ not being ignored (maybe we need to write a .gitignore file at the same time?).

There's an issue with the current version of rubygems, and downgrading to 2.1.11 fixes those errors. See https://github.com/bundler/bundler/issues/2784 for more information.

In addition, ```source :rubygems``` was deprecated some time back so just fixing that up.