Check assets:clobber is defined before enhancing (#71)

rails/cssbundling-rails@a749ca2 suggests that "Sprockets is not a dependency", so we should not
assume that the assets:precompile AND assets:clobber tasks will be defined. If it's not
there, we should just skip enhancing it.

Related to: rails/cssbundling-rails#59

lib/tasks/cssbundling/clobber.rake

@@ -5,4 +5,6 @@ namespace :css do
   end
 end
 
-Rake::Task["assets:clobber"].enhance(["css:clobber"])
+if Rake::Task.task_defined?("assets:clobber")
+  Rake::Task["assets:clobber"].enhance(["css:clobber"])
+end