Travis CI: reduce hardcoding of Python 3 versions

[cclauss]

Summary

Python 2.7 needs pil but all other currently supported versions of Python need Pillow instead.

Related Issues

PR Overview

• This PR requires new unit tests [y/n] (make sure tests are included)
• This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• This PR is backwards compatible [y/n]
• This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

[cclauss]

Tensorflow is not yet ready...

$ pip install tensorflow==1.7
Collecting tensorflow==1.7
Could not find a version that satisfies the requirement tensorflow==1.7 (from versions: )
No matching distribution found for tensorflow==1.7
The command "pip install tensorflow==1.7" failed and exited with 1 during .
Your build has been stopped.