add User Agent as it is required by the github API

All API requests MUST include a valid User Agent string.
This prevents us from the "I cannot access the API of github." error message.

See http://developer.github.com/v3/#user-agent-required for details:

Requests with no User Agent string will be rejected. If you are hitting the API without authentication, we ask that you add some kind of identification to the UA header value. This is so we can contact you if there are problems.