add CircleCI support

[erezrokah]

Fixes #16

See pr here erezrokah/gatsby-netlify-cms-aws#10

I'm still missing the github status updates, but I'm assuming it is due to this:

(Still waiting for the bot response)

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).

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[erezrokah]

I signed it!

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CLAs look good, thanks!

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