Replacing drush with bundled install.sh

We can't keep using Drush 5.7 forever, this removes the dependency until we can add official Backdrop support to Drush.

https://github.com/backdrop/backdrop-issues/issues/118
https://github.com/backdrop/backdrop/pull/91
Merge pull request #1 from quicksketch/patch-1

Replacing drush with bundled install.sh