Only test indentation on pull requests.
Merge pull request #117 from luca-heltai/travis_ci_indent_check

Only test indentation on pull requests. This closes issue #89, where we discussed about possibly testing with Travis CI the indentation of the pull request.

Since most of the pull requests are failing anyway on Travis (time limit for a pull request is 25 minutes, and we hit it pretty regularly), this pull request configures Travis CI to only check indentation in pull requests, and do the full build only when the pull request is merged on master.