Bump to github 0.3 which fixes proxy install issue

The latest github cookbook, version 0.3, has a fix that picks up the
http_proxy/https_proxy environment configuration during network calls.
This fixes #106 where Berkshelf API would not install when using a
proxy.
Merge pull request #148 from amaltson/github-asset-fix

Bump to github 0.3 which fixes proxy install issue