fix for coverage badge

Hi, I saw that we hadn't received any builds on https://coveralls.io/r/peter-murach/github so I looked into it and noticed a couple things:

WebMock was blocking the API POST to coveralls.io, and while in the past we had recommended adding "coveralls.io" to the allowed domains list, I went ahead and added auto-detection to the gem:

https://github.com/lemurheavy/coveralls-ruby/commit/8f02a494548fa21467ecdc2a2ec31846d20d9393#L1R32

Also, since the suite is running both specs and features, I enabled coveralls for specs only (since it's 95% vs Cucumber's 74%); we're working on support for aggregate test results, but not sure yet if it's possible.

Thanks!