Update Travis CI build and run/lint/verify examples

[jbcrail]

While working on the newest release, I took a prior notebook verification tool that I wrote and combined it with @ceball's previous work in #463 to improve our Travis build and testing of the example notebooks. Here is a summary of the changes:

• We can now lint, verify, and execute any notebook. The lint and verify tools are used in Travis CI, while the notebooks can be executed locally since the full datasets are still required. All notebooks are tested using the examples environment.
• I refactored the Travis CI scripts, which can also be ran locally. The Xcode environment variable is no longer shown during Linux builds. I heavily borrowed Bokeh's CI setup.
• I fixed several notebooks using the new verify mode (broken URLs, unused imports, etc).

We could later split the examples/nb CLI into a separate repo to share between projects.

Related to #407, #463

[jbednar]

Thanks. This is a clear improvement, so I'll merge it, but I hope that nearly all of this can be deleted very soon. We already have prototypes of code to do most of this in ways that don't have to be duplicated across projects, and we're maintaining so many simultaneous projects that I don't want to have any more boilerplate per project than we absolutely have to.