Rollback mocha / shoulda-matchers

As @quark-zju [observed](https://github.com/gregbell/active_admin/pull/1791#issuecomment-15128011):
>  `mocha` is required by `bourne`, which is in turn required by `shoulda-matchers`. After explicitly specifing `shoulda-matchers` to `1.5.0`, all tests passed.

The problem is caused by `shoulda-matchers` 0.5.1 and `mocha` 0.13.3 This can be locally reproduced simply by running `bundle update`. See [this gist](https://gist.github.com/Daxter/5199322) for a backtrace and gem versions.

- [Travis build before this PR](https://travis-ci.org/gregbell/active_admin/builds/5633564)