Remove dependency on pycurl in apt_repository

This pull request removes the dependency on `python-pycurl` for `apt_repository`, by replacing the pycurl calls with `fetch_url` from `module_utils/urls.py`

pycurl was historically used to verify SSL certs, and now that fetch_url exists and can handle this, there probably is no reason to keep the dependency on pycurl.

I've not added the `validate_certs` attribute, since we were forcing SSL cert verification anyway previously.

I've run the test_apt_repository integration tests on an ubuntu 12.04 box, and everything appears to be passing.