Upgrade python/httplib2 to v0.18.1 am: f6b4dcdd5a am: 312ef219b4 am: 8cef85bf28 am: 43ecfa8e0c am: 8baad241a1

Change-Id: Iaa600b759b44a4b7c7a0b79b5c3f634da5073a4b