fix build error if resource directory does not exits

This bug is introduced by 064672443d

% motion create hello && cd hello
% rm -rf resources
% rake
rake aborted!
No such file or directory @ dir_initialize - ./resources
/Library/RubyMotion/lib/motion/project/template/ios.rb:51:in `open'
/Library/RubyMotion/lib/motion/project/template/ios.rb:51:in `entries'
/Library/RubyMotion/lib/motion/project/template/ios.rb:51:in `block in pre_build_actions'
/Library/RubyMotion/lib/motion/project/template/ios.rb:50:in `each'
/Library/RubyMotion/lib/motion/project/template/ios.rb:50:in `pre_build_actions'
/Library/RubyMotion/lib/motion/project/template/ios.rb:66:in `block (2 levels) in <top (required)>'
/Library/RubyMotion/lib/motion/project/template/ios.rb:121:in `block in <top (required)>'
Tasks: TOP => build:simulator
(See full trace by running task with --trace)

lib/motion/project/template/ios.rb

@@ -48,6 +48,7 @@ namespace :build do
     # TODO this should go into a iOS specific Builder class which performs this
     # check before building.
     App.config.resources_dirs.each do |dir|
+      next unless Dir.exists?(dir)
       Dir.entries(dir).grep(/Resources/i).each do |basename|
         path = File.join(dir, basename)
         if File.directory?(path)