Remove rcov support

As part of dropping Ruby 1.8 support, we can remove this cruft.
Add a test task for the lib/ directory and configure coverage for it
Remove platform requirement of simplecov in the Gemfile

Since simplecov is needed for all Ruby versions > 1.8
Merge branch 'remove-rcov' of https://github.com/ferrous26/Markus into ferrous26-remove-rcov