README: also mention CentoOS and RHEL in instructions
README: also mention CentoOS and RHEL in instructions

The latest supported version of Fedora is now 24, and python3 is
supported on all current versions, so drop references to old Fedora
from the README.

Fixes #47.

Awesense/python-systemd,keszybz/python-systemd,systemd/python-systemd,keszybz/python-systemd,Awesense/python-systemd,Awesense/python-systemd,systemd/python-systemd