add option to maintain the `latest` version of composer

[igalic]

we should provide an option to keep our composer installation current.

we could run self-update, but that feels hacky.

[scottsb]

Isn't this what the auto_update flag does that you inherited from willdurand/puppet-composer?

[igalic]

i think the trouble with auto_update is that it will be run as the user we want it to run, while composer.phar itself may be owned by root.