Adds '!default' to variables so we can override them, as done in the
compass port of bootstrap. No need for generators or anything, just set
the variables you want to override before importing bootstrap with
…re it (gemspec dependency are not automatically required). When used from an engine, this ended up in weird races conditions when sass-rails wasn't present in main_app's Gemfile (which requires it by default).
Should fix sferik/rails_admin#759