Set up Travis CI[1] to use xctool[2]

Requires I…

- share out my schemes
- disable Xcode's 'Find Implicit Dependencies'
- add CocoaPods targets as explicit build dependencies
- install all of my dependencies (CocoaPods, xctool) directly from my
  repo (see `script/bootstrap`). Note: using an older version of xctool
  because that seems to be the only version that works for both me and
  Travis CI.
- tweak my xctool command with environment configs that should not be
  required

[1]: https://travis-ci.org/
[2]: https://github.com/facebook/xctool