Add global requirements for composer

Uses @johnpbloch's suggested method for setting this up as a first go.

https://gist.github.com/johnpbloch/6264213
Mockery is at 0.8.0, not 1.8.0
Remove previous composer config for phpunit and mockery

Now that things are global, this should work?
Set the bin-dir config value after adding requirements
Silence the composer commands that just set things up
Remove custom composer.json setup in favor of new global config

Also removes Hamcrest, as it is not needed or useful on its own.

See #129 for full discussion
Merge pull request #129 from 10up/129-global-composer

Make use of the new global composer command