Fixing sass-rails issue

Fixing the assets precompile issue, which was caused by sass-rails 4.0.1... forcing it to be > 4.0.3 fixes the issue. 

Also added in a couple of gems for improving image serving in production. In order to install do 'sudo apt-get install libjpeg-dev libpng-dev libtiff-dev libgif-dev libwebp-dev'