add CircleCI support

[erezrokah]

Fixes #16

You can test it by installing directly from my branch:

yarn add --dev "https://github.com/erezrokah/lighthousebot#add_circle_ci_support"

Also I think there is a bug in the Travis support. TRAVIS_PULL_REQUEST_SLUG should be TRAVIS_REPO_SLUG (https://docs.travis-ci.com/user/environment-variables/). I think in case of forks TRAVIS_PULL_REQUEST_SLUG will contain the information of the fork, where you actually want to update the current repo status (I'm not sure since I don't use Travis).

[erezrokah]

Is there anything missing on my end to get this PR merged?

[fernandopasik]

@addyosmani please could you help us with this one?