travis: drop python 2.7/3.4, add 3.7
travis: drop python 2.7/3.4, add 3.7

Signed-off-by: Niklas Koep <342d5290239d9c5264c8f98185afedb99596601a@gmail.com>

nkoep/pymanopt,pymanopt/pymanopt,pymanopt/pymanopt,nkoep/pymanopt,nkoep/pymanopt