Enable Travis CI for webcompat.com repo

I've set up the private env vars on the Travis side and enabled the webhook for the repo.

Other than adding .travis.yml, this PR allows us to load certain values for config.py from environment variables, some private ones and some defined in .travis.yml.

Also adds a status image in README.

r? @karlcow for config.py changes
r? @vladikoff  for .travis.yml (if you don't mind)