Require a version of httplib2 that includes the DigiCert root certs.

After they replaced their SSL certificates due to the HeartBleed issue,
the certificate for *.trello.com is signed by a chain that has the
DigiCert key at its root. Until version 0.9, httplib2 did not include
that cert in its cacerts.txt file.
Merge branch 'master' of https://github.com/bodylabs/py-trello into bodylabs-master

Conflicts:
	setup.py