Fixes geerlingguy#17: Allow configuration of drush git clone 'force' …

…parameter.

Merge pull request geerlingguy#14 from thom8/force_update

PR geerlingguy#14: Force git update when composer.lock is modified.

PR geerlingguy#13: Slight adjustment for drush composer update.

Fixes geerlingguy#11: Make Drush composer-based install CLI options c…

…onfigurable.

Better syntax formatting.

Fix for Issue geerlingguy#6: Get Drush to install on Debian Wheezy.

Fixes geerlingguy#6: Drush master doesn't install on Debian Wheezy.

Add automated tests to Drush role.