remove #stub and #mock
Don't override let definitions with argument matchers

see https://github.com/rspec/rspec-mocks/issues/102

ArgumentMatchers (together with ExampleMethods) were
included into the ExampleGroup instance's metaclass.
This caused argument matcher methods such as boolean()
or hash_containing() to override let definitions with
the same name.

This commit fixes this bug by including
ArgumentMatchers into the example group's class and
not into the metaclass.