Drop Python 3.4 support [travis]
Drop Python 3.4 support [travis]

pandas no longer has wheels for Py-3.4, so we can't pip-install.
We require pandas for serialization/deserialization, which is an
essential (not optional) feature. Therefore we can no longer
support 3.4. Also, MDTraj doesn't test against 3.4 anyway.

dwhswenson/contact_map,dwhswenson/contact_map