Travis: Refactor CI files, use container infrastructure.

 * Split build steps to utilize the Travis build lifecycle.
 * Move shell code from `.travis.yml` into Bash files in `.ci/`,
   one file for each step of the Travis build lifecycle.
 * Use configuration variables in `.travis.yml` to change
   build behavior (e.g. build 32-bit with `BUILD_32BIT=ON`).
 * Keep all configuration in environment variables in
   `.travis.yml`. In scripts, concatenate environment variables
   according to configuration to change to different behavior.
 * Add GCC 5 builds for Linux.
 * Use Travis's caching feature [1] for third-party dependencies
   and pip packages.
 * Allow failures MSan, as the errors it reports have to be
   fixed first.

Valgrind is still disabled, but can be enabled by setting
`env: VALGRIND=ON` for a job in `.travis.yml`.

[1] http://docs.travis-ci.com/user/caching

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