Configured SimpleCov to run on our code

This is a strongly-recommended step for all project teams
Others, first bundle install. Note that SimpleCov specifically
opts out of being "required" with the bundling so that it can assess coverage.

It is therefore explicitly required in our spec_helper.

Our current coverage reveals gaps in:

 - Seed (refactoring and testing already underway by Devin)
 - LocationLookupController (on our to-do list, also flagged by Flog)
 - BeersController (50% coverage; let's just test the other method)