Make use of the new global composer command

Composer just got a shiny new `global` command composer/composer@7311bc77e68b229dbb19f7bc12f2b8a3b17ab936 and it would be nice to use it to install global composer packages. I put together [a script that installs phpunit globally with the new command](https://gist.github.com/johnpbloch/6264213). We'd of course want to change almost all of it, but it's a decent starting point.