Fix travis failures on Ubuntu

[mikepurvis]

Travis was failing on Ubuntu on account of not being able to install the enchant package. Seems the base images were configured with apt having only ${dist}-security plus a ton of PPAs, and no default repos. This is baffling to me, but also easy to correct at build time.

I've also moved the builds from Trusty to Xenial, given that Trusty is EOL next month anyway.

[mikepurvis]

Going to disable the OS X builds, as they are failing for infrastructure reasons (too many open files).

[mikepurvis]

@wjwwood Does anything here offend you?

My intent is to (personally) run the current head from source for a few days, and then propose that 0.4.5 be released.

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A few issues noted— 

Getting the Unhandled config context options on every invocation. Need to check if #489 is a fix here. Done.

Getting some yaml-loading warnings on startup, fixed in #542.

Also, getting a bunch of /home/travis/virtualenv/python3.4.8/lib/python3.4/site-packages/sphinxcontrib/spelling/__init__.py:11: RemovedInSphinx20Warning: app.info() is now deprecated. Use sphinx.util.logging instead., but this looks like a problem with sphinxcontrib-spelling v4.20, which is the current one, released June 2018.

[wjwwood]

When you're ready to release, send me a direct email please, and I will help you.