Mocha 2.0+ / Minitest 5.19+ compatibility (#266)

* Mocha 2.0+ compatibility

The support for `require 'mocha/setup'` is long time deprecated and was
removed in Mocha 2.0:

freerange/mocha@642a0ff4

* Fix compatibility with Minitest 5.19+

The `MiniTest` was renamed to `Minitest`:

minitest/minitest@9a57c52

And the `MiniTest` constant is now loaded just when `MT_COMPAT`
environment variable is set:

minitest/minitest@a2c6c18

test/unit/cors_test.rb

@@ -4,7 +4,7 @@
 require 'rack/lint'
 require 'minitest/autorun'
 require 'rack/test'
-require 'mocha/setup'
+require 'mocha/minitest'
 require 'rack/cors'
 require 'ostruct'
 
@@ -21,7 +21,7 @@ def options(uri, params = {}, env = {}, &block)
   def_delegator :current_session, :options
 end
 
-module MiniTest::Assertions
+module Minitest::Assertions
   def assert_cors_success(response)
     assert !response.headers['Access-Control-Allow-Origin'].nil?, 'Expected a successful CORS response'
   end