Add test coverage badge

this will add the test coverage badge to feedbin. if you want to have this, there's just a few steps you need to follow:

1. go to [the code climate page for feedbin](https://codeclimate.com/github/feedbin/feedbin) and click "Set Up Test Coverage" in the sidebar
1. copy the `CODE_CLIMATE_REPO_TOKEN` value
1. go to [the travis page for feedbin](https://travis-ci.org/feedbin/feedbin) and click on the settings gear, then "Settings", then on that page "Environment Variables"
1. add the `CODE_CLIMATE_REPO_TOKEN` environment variable. (you can make it private, so the value won't print in the travis output)
1. merging this should trigger a travis build that reports test coverage to code climate and displays as a badge in the readme