Move Pingdom requests back into client class

This effectively reverses [3ae441](https://github.com/alphagov/backdrop-pingdom-collector/commit/3ae441) from the alphagov/backdrop-pingdom-collector repository and changes the tests to just mock the request method in the `Pindom` 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.