Add `requests` module as a dependency in travis.yml

In response to http://stackoverflow.com/questions/22271062/error-when-launching-cpp-coveralls

It seems `cpp-coveralls` doesn't properly declare [its dependency on the `requests` module](https://github.com/eddyxu/cpp-coveralls/blob/master/coveralls/report.py#L4), therefore it needs to be installed manually beforehand.