Copy Mocha bug fix.

A bug was fixed [1] in Mocha's integration with Test::Unit, but this
monkey-patching code was copied before the fix. We need to copy the
fixed version.

The bug meant that an unexpected invocation against a mock within the
teardown method caused a test *error* and not a test *failure*.

[1]
freerange/mocha@f1ff647#diff-5

activesupport/lib/active_support/testing/setup_and_teardown.rb

@@ -83,6 +83,8 @@ def run(result)
               begin
                 teardown
                 run_callbacks :teardown
+              rescue Mocha::ExpectationError => e
+                add_failure(e.message, e.backtrace)
               rescue Test::Unit::AssertionFailedError => e
                 add_failure(e.message, e.backtrace)
               rescue Exception => e