Fix Travis integration config option in .coveralls.yml.

"Another important option is service_name which allows you to specify where Coveralls should look to find additional information about your builds. This can be any string, but using travis-ci or travis-pro will allow Coveralls to fetch branch data, comment on pull requests, and more."
Merge pull request #17 from coursemology-collab/lowjoel/fix-coveralls-travis-config

Fix Travis integration config option in .coveralls.yml.