The problem installing SASS for Odoo is the installation of the proper versions of the corresponding components.

The following components should be installed:

1. Ruby Version 2.2 (my recommendation, it must be 2 or higher, default Ubuntu deb is 1.9!)

2. SASS Version 3.4.25 (as implemented by compass, must be >= than 3.3.4)

3. compass (requires SASS >= 3.3.0 and < 3.5)
   

4. bootstrap-sass
   

An easy way to accomplish this is the following:

Get the Brightbox repository to install the Ruby version you like:

$ sudo apt-get install software-properties-common
$ sudo apt-add-repository ppa:brightbox/ruby-ng
$ sudo apt-get update

Then install Ruby 2.2:

$ sudo apt-get install ruby2.2 ruby2.2-dev

After that you can install SASS 3.4.25:

$ sudo gem install sass -v 3.4.25

and finally install compass and bootstrap-sass:

$ sudo gem install compass bootstrap-sass

Done.

SASS is needed for certain website modules. Here is a further reading: https://github.com/OCA/maintainer-quality-tools/pull/465