Update source building instructions
Update source building instructions

* Only list Fedora 21+. Older releases are no longer supported by upstream, let alone for systemd development.
* Fedora now uses `pip` and `pip3`, not `pip-python` and `pip-python3`. By only documenting Fedora 21+, we can be consistent in the instructions.
* Use HTTPS for git (rather than HTTP).
Awesense/python-systemd,again4you/python-systemd,again4you/python-systemd,systemd/python-systemd,Awesense/python-systemd,keszybz/python-systemd,Awesense/python-systemd,keszybz/python-systemd,systemd/python-systemd