rails4/ruby2.2: explicitly require test-unit

Change-Id: Ia9be2e8063eb4ab088ca6b8afc949d3e375f5240
Reviewed-on: https://gerrit.instructure.com/59874
Tested-by: Jenkins
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

spec/spec_helper.rb

@@ -22,6 +22,7 @@ rescue LoadError
 end
 
 require 'securerandom'
+require 'test/unit' if RUBY_VERSION >= '2.2.0'
 
 RSpec.configure do |c|
   c.raise_errors_for_deprecations!