Fixes load path issues with ruby-based git credential helpers

Otherwise, we get errors like this (with boxen, e.g.):

```
/opt/boxen/bin/boxen-git-credential:23:in `require': cannot load such file -- boxen/config (LoadError)
        from /opt/boxen/bin/boxen-git-credential:23:in `<main>'
Username for 'https://github.com':
```
Merge pull request #1609 from alindeman/patch-1

Fixes load path issues with ruby-based git credential helpers
Merge branch 'master' of git://github.com/imathis/octopress

* 'master' of git://github.com/imathis/octopress:
  Fixes load path issues with ruby-based git credential helpers
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