Upgrade Travis testing from Python 3.5 to 3.6 (#8148)
Upgrade Travis testing from Python 3.5 to 3.6 (#8148)

To match [setup.py](https://github.com/fchollet/keras/blob/master/setup.py#L34).  There are a number of questions raised in #7297 about Python 3.6 support so let's test that version.
keras-team/keras,keras-team/keras