Enforce Ruby version

The pessimistic operator makes it possible to install Suspenders on Ruby
2.1.2 but then run Suspenders and have it not bundle because the Gemfile
is restricted to Ruby 2.1.1. This change enforces an exact Ruby version.
Remove SimpleCov

* SimpleCov was returning zero status even when build failed.
* Remove SimpleCov so that we get real build results on Travis.
* The team was reporting not regularly reviewing the coverage stats.