Log the language of repositories to metrics
Switch repo over to Docker infrastructure
Normalize strings used in metrics

Librato does not like spaces and such, and this will look better too
(with everything lowercased).
Handle nils when normalising strings
Turn nils into "empty"

Otherwise we would have metrics such as “travis_yml.language.”.
Tracking them is still useful, I think, but this makes it clearer.
Prevent exceptions in metrics code from raising

Since this happens late in the request process and there are no
transactions, raising would mean that the request is re-queued, but the
builds are already created, causing an infinite loop.