Add a simple test for plot_array_distribution.
Uncomment matplotlib in .travis.yml
Use Agg backend.
Try xvfb
Test using savefig.
Add a DISPLAY variable.
Remove the savefig.
Add a comment.
Add some comments to .travis.yml.
Merge branch 'master' into feature/simple-plotting-test
Skip plotting test if import fails.

This requires moving `test_plotting.py` outside of the `distarray/plotting/` directory because `distarray/plotting/__init__.py` imports everything from plotting.py, causing the import of `test_plotting` to fail regardless of what's in it (if you're missing matplotlib).
Merge pull request #364 from enthought/feature/simple-plotting-test

Simple plotting test