Fix `mocha/setup` deprecation warning

Prevents this warning when running the tests:

    Mocha deprecation warning at ... test/test_helper.rb:5:in `require':
    Require 'mocha/test_unit', 'mocha/minitest' or 'mocha/api' instead of 'mocha/setup'.

Requiring `mocha/setup` was deprecated in 1.10.0:

https://github.com/freerange/mocha/blob/master/RELEASE.md#1100

test/test_helper.rb

@@ -2,7 +2,7 @@ require 'rubygems'
 require 'minitest/autorun'
 require 'webmock/minitest'
 require_relative 'lib/webmock_extensions/last_request'
-require 'mocha/setup'
+require 'mocha/minitest'
 require 'pry'
 
 $LOAD_PATH.unshift(File.dirname(__FILE__))