Fix nightly builds not being recognised as such in setup_build 

It appears that nightly builds aren't getting published:
http://deb.theforeman.org/pool/wheezy/nightly/f/foreman/ (just stable releases)

I see in the logs that setup_build doesn't think they're nightly builds:
http://ci.theforeman.org/job/packaging_build_foreman_individual-DEB/1825/console

<pre>
+ ../../buildscripts/build.sh wheezy64 nightly
Cloning into /var/lib/workspace/workspace/packaging_build_foreman_individual-DEB/debian/wheezy/wheezy64/foreman...
Already on 'develop'
not nightly: skipped setup_build
</pre>

Hopefully this'll do the trick.