Rename Expectation.method_name to Expectation._method_name
Rename Expectation.called to Expectation._called
Use 'no' instead of 'false' for Expectation.called value.
Now uninstall mocked methods in reverse.

A second mock of the same method name on the same object will
remember the first mock method as the original method.  It is the
first mock that remembers the actual original method.  This means
we have to uninstall first-in-last-out.
Move build_not_called_error() to Expectation.build_errors().
Rename build_errors()/build_all_errors() to find_errors()/find_all_errors().
Rename state_new_expectation() to create_expectation().
Install 'GitHub for Mac' via Vagrantfile.