Update README to use newer syntax
Update README to use newer syntax

Update the example code on the README to use the .get() function rather
than register_uri('GET'). This is the first thing most people see and we
want to make that look as clean as possible.

Change-Id: I87e0d40d76f70acfbe8ba9686a5ea5a6b0d73f85

stackforge/requests-mock,jamielennox/requests-mock