Remove Zenoss from INSTALL_REQUIRES.

This is to allow trunk ZenPacks to be tested on current stable Zenoss
versions.

Reviewed by gstreiff and jcausey.

git-svn-id: http://dev.zenoss.com/svnint/trunk/core/zenpacks/ZenPacks.zenoss.XenMonitor@74543 f5df01ce-0320-0410-9ed9-ddcd13c732cc