Elastic beanstalk - installing SASS and Compass (Symfony)

I am trying to deploy symfony 2 application on Elastic Beanstalk. For assetic to work I need sass and compass installed. I created a provisioning script:

packages:
   rubygems:
       sass: '3.4.9'
       compass: '1.0.3'

It works, since in the logs I get:

2015-01-21 10:07:03,800 [DEBUG] Checking to see if sass-3.4.9 is already installed
2015-01-21 10:07:04,079 [INFO] sass-3.4.9 is already installed, skipping.
2015-01-21 10:07:04,080 [DEBUG] Checking to see if compass-1.0.3 is already installed
2015-01-21 10:07:04,363 [INFO] compass-1.0.3 is already installed, skipping.

When I ssh into the EC2 instance however, I cannot run neither (which says they are not installed, so does gem --list). Any idea how to hande this?