This should fix the following case which uses "HTBResources.bundle" as resource.
```
% rake
ERROR! An iOS application cannot be installed if it contains a directory called `resources'. Please rename the directory at path `vendor/Pods/Resources/HTBResources.bundle' to, for instance, `vendor/Pods/Resources/assets'.
````
it cause an error If app.resources_dirs has nested array
app.resources_dirs << ['Images']
```
% rake
rake aborted!
no implicit conversion of Array into String
/Library/RubyMotion/lib/motion/project/template/ios.rb:51:in `exists?'
/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:67:in `block (2 levels) in <top (required)>'
/Library/RubyMotion/lib/motion/project/template/ios.rb:122:in `block in <top (required)>'
Tasks: TOP => build:simulator
(See full trace by running task with --trace)
```
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)
