Add test coverage; account for no artifacts on CI

Drop coverage a bit for JRuby and Rubinius because they
don't generate the same Coverage as CRuby

test/test_helper.rb

@@ -1,5 +1,16 @@
 require 'bundler/setup'
 
+begin
+  require 'simplecov'
+  # HACK: till https://github.com/colszowka/simplecov/pull/400 is merged and released.
+  # Otherwise you may get:
+  # simplecov-0.10.0/lib/simplecov/defaults.rb:50: warning: global variable `$ERROR_INFO' not initialized
+  require 'support/simplecov'
+  AppCoverage.start
+rescue LoadError
+  STDERR.puts 'Running without SimpleCov'
+end
+
 require 'timecop'
 require 'rails'
 require 'action_controller'