Add rubocop checks to test suite run.

This is the first step into working on fixing issue #13, I tied rubocop into the build and silenced all the warnings that where currently present. If/when this gets merged we can enable and fix the checks causing the unnecessary assignment warnings and prevent any from reappearing.

I also accidentally included a fix to change the gem source to the modern preferred form. If this is a problem I can revert that change and keep the rest of the PR.