fix a bug where nested array cannot be handled
fix a bug where nested array cannot be handled

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)
```

beni55/RubyMotion,incbee/RubyMotion,beni55/RubyMotion,HipByte/RubyMotion,incbee/RubyMotion,katsuyoshi/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,jjaffeux/RubyMotion,beni55/RubyMotion,HipByte/RubyMotion,katsuyoshi/RubyMotion,hboon/RubyMotion,katsuyoshi/RubyMotion,hboon/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,jjaffeux/RubyMotion,HipByte/RubyMotion,katsuyoshi/RubyMotion,beni55/RubyMotion,hboon/RubyMotion