Display coverage stats when developing locally

The Coveralls gem doesn't display coverage statistics directly in the
console when run locally. Get around this by using SimpleCov directly
when not in Travis so we get the actual coverage stats rendered in the
`coverage` directory, making it quicker to write tests and see how
much coverage has changed.

Author: sds

spec/spec_helper.rb

@@ -1,6 +1,12 @@
-# Bootstrap code coverage instrumentation. This must happen first!
-require 'coveralls'
-Coveralls.wear!
+if ENV['TRAVIS']
+  # When running in Travis, report coverage stats to Coveralls.
+  require 'coveralls'
+  Coveralls.wear!
+else
+  # Otherwise render coverage information in coverage/index.html and display
+  # coverage percentage in the console.
+  require 'simplecov'
+end
 
 require 'overcommit'
 require 'tempfile'