don't get shouty about rcov not being installed

just do it if someone tries to run the rcov task

lib/tasks/coverage.rake

@@ -63,5 +63,11 @@ begin
   end
 
 rescue LoadError
-  "You must install the 'rcov' gem to use rcov_rails\n\tsudo gem install rcov"
+
+  namespace :test do
+    task :coverage do
+      puts "You must install the 'rcov' gem to use rcov_rails\n\tsudo gem install rcov"
+    end
+  end
+
 end