Drop Python 2.6 support from Travis.

As far as I'm aware, the code is still valid under 2.6, but the
'buffer' keyword argument to unittest.main doesn't exist there.
Since this will only be run on 2.7+, drop the 2.6 requirement that
we don't need.

Change-Id: I761dc9bb59619ccb405ca96f6644577a5f3a2450