remove quotes around `ref` as they will break `:git` locations (at least...

... on win7 / msysgit)

The single quotes around `ref` break the `:git` locations (on windows /msysgit) at least, see [this comment](https://github.com/RiotGames/berkshelf/issues/601#issuecomment-18823619) in #601

This PR removes the single quotes which is consistent with other occurrences of `ref` being passed to `git(*cmd)` in the same file