Support Coveralls Pro (and Travis Pro)

Fix kt3k/coveralls-gradle-plugin#4

When using Coveralls Pro, we must set the `service_name` to `travis-pro` and also supply a field called `repo_token` which has a private token retrieved by a user from coveralls.io.

Support this behavior by

1. Adding a `repoToken` field to the `ServiceInfo` class.
2. Add support for the `COVERALLS_REPO_TOKEN` environment variable, which is propagated to the new `repoToken` field.
3. For Travis-CI, set the service name to `travis-pro` if the `COVERALLS_REPO_TOKEN` environment variable is present.
4. Modify the `Report` object to add the new field.
5. Update all the tests and add a few new ones.