fix for compass filter

Fix for exception in windows environment

 [Assetic\Exception\FilterException]                                          
  An error occurred while running:                                             
  "C:/Ruby200-x64/bin/ruby.exe" "C:/Ruby200-x64/bin/compass" "compile" "C:\Us  
  ers\Developer\AppData\Local\Temp" "--config" "C:\Users\Developer\AppData\Local\  
  Temp\assB843.tmp" "--sass-dir"  "--css-dir"  "C:/Users/Developer/AppData/Loca  
  l/Temp/assB844.tmp.scss"                                                     
                                                                               
  Error Output:                                                                
  You must compile individual stylesheets from the project directory.