Refactor RebuilderJob#perform method

[chrismytton]

Problem

Currently the RebuilderJob#perform method is a big procedural mess. Making changes to the method isn't too difficult, but adding tests for those changes is near impossible.

Proposed solution

Split this method up into discrete classes that are each well tested. We probably need at least the following concepts modelled as :

• Cloning ep-data, running bundle install and running the ep-data rake tasks and capturing the output. See https://github.com/everypolitician/rebuilder/issues/45 and https://github.com/everypolitician/rebuilder/issues/52.
• Processing the output to remove color codes, Morph API tokens, only taking the first 64k of output etc. https://github.com/everypolitician/rebuilder/issues/43
• The resulting pull request's title and body.

[chrismytton]

Some nice initial wins from extracting the output cleaning into a separate class in https://github.com/everypolitician/rebuilder/pull/64.

Next up I think is #43 https://github.com/everypolitician/rebuilder/issues/45, which has been started over in #63.