coveralls.io internal server error handling

At time of writing, coveralls.io is [suffering from connectivity issues](https://twitter.com/CoverallsApp/statuses/444365955766837248). In this case Coveralls plugin is failing with JSON parsing error (see https://travis-ci.org/levkhomich/akka-tracing/jobs/20885510). This pull request fixes the problem by explicitly handling response code 500 with corresponding server message.