Optionally lock down travis.coffee

Allow users to set HUBOT_TRAVIS_TOKEN, which is used to compare against a
nonce sent with every Travis payload. The nonce is a SHA256 hash of your
project's owner/repo combination and Travis Token. If HUBOT_TRAVIS_TOKEN
is unset, travis.coffee will continue to accept any POST request
regardless of where it actually came from.

Signed-off-by: David Celis <me@davidcel.is>