packaging: spec: fix fedora build requirements

In Fedora < 21  and EL >= 6 python-setuptools-devel was provided
by python-setuptools.
In Fedora 21 such package is not provided anymore.

Also fixed URL for Source.

Change-Id: I2b1ff672bb6dd4adaf3695bb1ab87d0f2f0f8d64
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>