Add a gitdeploykey provider.

The purpose is the reuse the deploy key that we already have in our
system.

This is just the barebones version to test it out. The proper version
could automatically install the key that we use automatically in a
follow-up patch.
Move provider/error requires to spec_helper.
Merge pull request #98 from travis-ci/mm-heroku-git-deploy-key

Add a gitdeploykey provider.
Try with heroku-api locked to 0.3.16.
Set up the git-ssh command for gitdeploykey strategy.

An unpleasant hack right now, to try out whether this kind of deploy
works at all.