Remove Psych from Gemfile

The reason this was here was to resolve Bundler / Psych double loads

    `<class:Parser>': superclass mismatch for class Mark (TypeError)

https://github.com/carlhuda/bundler/issues/2068

The issue has been fixed in Rubygems >= 2.0 and Bundler >= 1.3.0, which
we should all be using now.