Test and update CI for matplotlib 2.0.0

Matplotlib 2.0.0 was just recently released. As this is a major release, the inline ploting backend needs to be tested to ensure that it works correctly, and the CI needs to be updated to use this version instead of 1.5.3.

As we are approaching the 0.7.0 release for Zeppelin, I have marked this as a fix for 0.7.1 in case it is not out in time.