Use github API for updates

This is presumably better.

Both git strategies have the same API, and so you can switch between them. The git clone one will work for non-github repos, which is a reason to keep it around.

Test coverage on the github api strategy and shared code is good, but the clone strategy is awkward to test on CI because of things like SSH keys, mocks, etc. However, it can be tested locally by enabling it in git_strategy_spec.rb.

One known issue is that if you run bundle from within bundle, it bundles in the current directory, not the specified one. This isn't a problem with the installed gem, but is the reason that the Updater module isn't tested.