Instead of just caching the third-party build output, cache the full
build directory. Always run make to ensure that updated dependencies
are downloaded.
* 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
