More stable dev compilation (thread safety)

I've had a lot of trouble working with sass-asset-pipeline in dev due to intermittent compilation failures such as: "NoMethodError: undefined method `name' for nil:NilClass" and various ArrayIndexOutOfBoundsExceptions.

I seem to have traced these to the SASS/Compass/JRuby compiler scripts not being thread safe.  I haven't had a chance to work out where the thread safety issue is but instead I have created a patch to only run one SASS compilation at a time.

I've also fixed a method signature and upgrade the version of SASS slightly (not a major version).