Infrastructure updates notice from Astropy Project

Hello from Astropy Project!

The following updates to your package might be necessary to ensure compatibility with the latest stable version of Astropy:

- [ ] MPLBACKEND is now set to Agg in `ci-helpers`, packages expecting interactive plotting should override it in `.travis.yml`
- [ ] Astropy 3.1 is not compatible with Numpy <1.13 versions. If you want to keep testing these older Numpy versions, please use ASTROPY_VERSION=3.0 or ASTROPY_VERSION=LTS in your Travis CI matrix.
- [ ] Add `sphinx-astropy` as a package dependency if you are using `astropy-helpers` 3.1 or later. Otherwise, your documentation build (e.g., on ReadTheDocs) might fail.
- [ ] If you are using `six` that is bundled with Astropy, please consider using the standalone `six` package instead.

If these are no longer applicable to you, please close the issue.

*This is an automated issue for packages that opted in for automated astropy-helpers update. If this is opened in error, please let @pllim know.*

xref astropy/astropy-procedures#108

Review the repository's .travis.yml, dependency configuration, and documentation build settings against each checklist item. Determine which Astropy compatibility updates apply, then verify the relevant CI and documentation configuration; if none apply, close the issue as requested.

Written by the indexing model from the issue text.