Install Python 3.3 from the Deadsnakes PPA. As this doesn't have pip,
install it manually into ~/.local.
~/.local/bin is apparently in Travis's default PATH, meaning "pip"
doesn't refer to Python 2's pip anymore, but to the manually
installed Python 3 version. Updated the scripts to use version-
suffixed executable names (e.g. pip2.7).
Set CC=cc to use system's default compiler when installing Python
modules, as gcc on OS X had a problem with compiling one of the
dependencies of the Neovim Python module.
* 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
