Deprecate requiring 'mocha/setup'

By forcing people to require the explicit integrations for Test::Unit or
Minitest, we will be able to provide more useful feedback if the
integration fails for some reason.

Note that requiring 'mocha/setup' was removed from the README in the
v1.0.0 release [1].

[1]: 1d6b08f

lib/mocha/setup.rb

@@ -1,4 +1,9 @@
 require 'mocha/integration'
+require 'mocha/deprecation'
+
+Mocha::Deprecation.warning(
+  "Use `require 'mocha/test_unit'` or `require 'mocha/minitest'` instead."
+)
 
 module Mocha
   def self.activate