fixed UTF-8 error on Mac OSX

After updating Ruby and Compass (might be even some more dependencies that I am not aware of) I faced an error regarding to a 'missing_method'. It seems like that when using en_US.UTF-8 as part of the locale python is passing string with 'u' and ruby cannot find the correct method, e.g u'project_path' instead of 'project_path' 

The errors message was:
compass: subprocess had error: stderr=NoMethodError on line ["264"] of
/Users/USERNAME/.rvm/gems/ruby-2.0.0-p247@global/gems/compass-0.12.2/lib/compass/configuration/inheritance.rb:
u

Using:
OSX 10.8.5
Python 2.7 
Ruby 2.0.0p247
Compass 0.12.2
Gem: 2.0.6


Using string cast seems to fix the issue.
I am not sure that that's the best way to do that, but it worked for me at the moment.