Fail fast if test-unit/minitest integration fails

If someone is explicitly requiring "mocha/test_unit" or "mocha/minitest"
then the implication is that they know which test framework they are
trying to integrate with. If the Mocha integration fails (because the
test framework hasn't been loaded) then we should raise an error.

Closes #229.

lib/mocha/minitest.rb

@@ -1,9 +1,6 @@
 require 'mocha/ruby_version'
 require 'mocha/integration/mini_test'
-require 'mocha/deprecation'
 
 unless Mocha::Integration::MiniTest.activate
-  Mocha::Deprecation.warning(
-    "MiniTest must be loaded *before* `require 'mocha/minitest'`."
-  )
+  raise "MiniTest must be loaded *before* `require 'mocha/minitest'`."
 end

lib/mocha/test_unit.rb

@@ -1,9 +1,6 @@
 require 'mocha/ruby_version'
 require 'mocha/integration/test_unit'
-require 'mocha/deprecation'
 
 unless Mocha::Integration::TestUnit.activate
-  Mocha::Deprecation.warning(
-    "Test::Unit must be loaded *before* `require 'mocha/test_unit'`."
-  )
+  raise "Test::Unit must be loaded *before* `require 'mocha/test_unit'`."
 end