Update Composer packages on every provision

In #129, we moved from a composer.json to a global configuration
for packages such as phpunit. In the current state, changing the
version numbers for these packages only modifies new boxes.

Instead, we can check for and install Composer separately from
a self update and a global update. This allows version changes
to be applied with a standard `vagrant provision`.
global bin-dir is required for Composer packages
Upgrade PHPUnit to 4.3.x
Remove duplicate config of Composer bin-dir
Merge pull request #492 from Varying-Vagrant-Vagrants/composer-provision

[PR] Allow Composer packages to update on `vagrant provision`