canonical-certification-client needs better dependencies

I added the ppa:checkbox-dev/ppa to a just-installed (from release image) 14.04 VM, to test the new hybrid graphics tests.

They didn't appear because, though the plainbox-provider-certification-client package was installed with the correct whitelist, the test definitions weren't there, because installing c-c-c didn't update plainbox-provider-checkbox (it was still at 0.4-1 from stock trusty).

I manually apt-get installed the provider, bringing it up to version 0.5~dev something, and then I saw the extra tests. I got another problem whcih I'll file separately, but this one is about having correct dependencies.

The canonical-certification-client package should depend on appropriate versions of other packages to ensure it works well, otherwise we will have to ask users to install a large set of packages, which is error prone; or worse yet, have them trial-and-error upgrading until they get the combination just right.