Feature/full coverage

I wasted a truly sad amount of time last night and today trying to get back to 100% code coverage. The holdouts are the two lines hit when trying to load a storage adapter without having the prerequisite gem. While I was able, with difficulty, to stub require to raise a LoadError, [SimpleCov doesn't like loading a file twice] (https://github.com/colszowka/simplecov/issues/60).

This PR easily gets us to 99.69% coverage, and modifies CONTRIBUTING.md to avoid setting a currently unattainable standard.

Finished in 2 minutes 19.57 seconds
709 examples, 0 failures, 4 pending
Coverage report generated for RSpec to /Users/aweiner/the_policy_machine/coverage. 642 / 644 LOC (99.69%) covered.