Move Pingdom requests back into Pingdom client class

This effectively reverses 3ae441 (from the
alphagov/backdrop-pingdom-collector) and changes the tests to just mock
the request method in the class. As far as I can see it was moved out of
the class to make testing easier but this can just be done in a slightly
different way.
Merge pull request #60 from alphagov/move-requests-in-pingdom

Move Pingdom requests back into client class