Also install zsh.
Added test/setup which downloads/extracts the test Ruby.

* Downloads a precompiled Ruby binary curtesy of RVM. Thanks @mpapis!
* Test Ruby is now stored in test/rubies/.
* Added a make task for test/rubies.
Reset rvm before runing tests.
Ensure that system ruby is installed.
Source chruby.sh, relative to chruby-exec.
Remove >/dev/null on the chruby call in chruby-exec.
Execute the shell using the same options as the current shell.
Always run the chruby-exec shell as an interactive/login shell.

* This will force the shell configuration to be loaded.
Override HOME with our own custom setup.

* Added .bashrc and .zshrc files that load test/helper.sh.
* This ensures that chruby-exec will load chruby.sh.
Override HOME in the chruby-exec tests.
Do not re-load chruby.sh, since it clobbers RUBIES.
Ensure that chruby-exec runs under bash.

* For some reason, zsh will run the command via /bin/sh, if there is no #!.
Added Travis CI badge.