Build documentation on TravisCI

[jhamman]

• Closes zarr RTD docs broken #1898 (remove if there is no corresponding issue, which should only be the case for minor changes)
• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including whats-new.rst for all changes and api.rst for new API (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[jhamman]

I think we've talked about doing this a number of times in the past. It seems that we've been running into a number of uncaught build failures (e.g. #1898, #1881). This is one way of testing that the documentation build will pass on read the docs. If someone has a better of idea of how to do this, I'm all ears.

[shoyer]

This is really good idea, thanks!

[shoyer]

The build is failing with an error message about upgrading ruby?!?

[jhamman]

Well isn't that nice. CI is a two edged sword! I'll take a look.

[shoyer]

It looks like the doc build fails when it hits a warning:

Warning, treated as error:
/home/travis/build/pydata/xarray/doc/api.rst:1: WARNING: Inline emphasis start-string without end-string.

We should either fix these warnings or stop them from failing the build :)

[fmaussion]

We should either fix these warnings or stop them from failing the build :)

It's really hard to get rid of all of them - I'd rather try option B ;)

[fmaussion]

Currently we have 106 warnings at doc build on my system

[jhamman]

I had the fail-on-warning option turned on. I agree we should drop that for now. After we merge this, I'll open an issue to track cleaning those up at some point. I don't have time to fix them right now.

[fmaussion]

Thanks @jhamman , this is very useful