Add tests for new license library call

As requested in https://github.com/escapestudios-cookbooks/newrelic/pull/139#issuecomment-63174882, here are some tests for the library call that determines licenses. I've also made a modification to the call itself, so it returns an appropriate value (`nil`) if `node['newrelic']` is not defined.
Merge branch 'martinb3-master'
RuboCop fix
version bump