Removing the post install script for the gulp branch. Per https://github.com/roots/roots/issues/1212. Removed bower as a dev dependency. Updated docs to add the new steps to globally install `bower`, and to run `bower install` and `gulp dev` after running `npm install.

Updated docs to add the additional steps because we have to manually run `bower install` and `gulp dev` after a fresh install now.

Removed bower as a dependency and update documentation with proper instructions