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, therefore it needs to be installed manually beforehand.
Merge pull request #1 from lukasgraf/patch-1

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