Fix requirements conflict

There is a new conflict during run.sh certsuite_venv setup. Requirements was installing moztest==0.5; however further along in the setup mozversion is requiring moztest>=0.6, causing this break:

  File "build/bdist.linux-i686/egg/pkg_resources.py", line 569, in resolve
pkg_resources.VersionConflict: (moztest 0.5 (/home/rwood/fxos-certsuite/certsuite_venv/lib/python2.7/site-packages/moztest-0.5-py2.7.egg), Requirement.parse('moztest>=0.6'))