Changed requirements versions to be minimum required version rather

than only accepted version.

Specifying requirements version numbers with == results in downgrading
systems which have a version of the required software greater (more
recent) than that listed.  By changing this to >= the requirements
should still be met without adversely affecting other software on the
system.  Explicitly setting one particular version number should only
be done when run within a virtualenv (which is probably a good idea
anyway, but not everyone will do it).