[Rails] Don't emit configuration files when installing extensions into an existing project.

lib/compass/commands/stamp_pattern.rb

@@ -72,7 +72,7 @@ Options:
       # all commands must implement perform
       def perform
         installer.init
-        installer.run(:skip_finalization => true)
+        installer.run(:skip_finalization => true, :skip_preparation => true)
         UpdateProject.new(working_path, options).perform if installer.compilation_required?
         installer.finalize(options.merge(:create => is_project_creation?))
       end

lib/compass/installers/base.rb

@@ -29,7 +29,7 @@ module Compass
       # Every installer must conform to the installation strategy of prepare, install, and then finalize.
       # A default implementation is provided for each step.
       def run(run_options = {})
-        prepare
+        prepare unless run_options[:skip_preparation]
         install unless options[:prepare]
         finalize(options.merge(run_options)) unless options[:prepare] || run_options[:skip_finalization]
       end