Rollup merge of #37584 - alexcrichton:travis, r=brson

Move all Linux/OSX CI infastructure to Travis

This commit configures our `.travis.yml` to test the full suite of tests we have
on Buildbot right now. A whole mess of docker images are added to the `src/ci`
directory which represent all the build environments for each configuration.
Each of these environments is then configured in `.travis.yml` to run on the
auto branch.

Note that the full matrix of tests aren't intended to be run on all PRs.
Instead, we continue to run only one entry in the matrix, forcing all others to
finish quickly. Only the `auto` branch should run the full matrix of builds.

Also note that the infrastructure hasn't quite been allocated yet to the
rust-lang/rust repository, so everything is disabled for now except for the one
build that happens on PRs. Once that infrastructure is allocated though we can
enable this and let it fly!

Notable modifications from the current test suite today:

* Android tests are run in rustbuild instead of the makefiles, for whatever
  reason I couldn't get the makefiles to work on Travis.
* A debuginfo test was updated to work with the current version of the Android
  NDK.
* Some dependencies in `mk/tests.mk` were fixed to allow running tests in
  parallel.