workaround for build error

When run "rake" on Yosemite, build error is occurred, like
```
$ rake
     Build ./build/MacOSX-10.10-Development
   Compile ./app/app_delegate.rb
      Link ./build/MacOSX-10.10-Development/menu.app/Contents/MacOS/menu
Undefined symbols for architecture x86_64:
  "_NSSquareStatusItemLength", referenced from:
      _MREP_FBAC5161392349B698908DFFEB2F205C in app_delegate.rb.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
rake aborted!
Command failed with status (1): [/usr/bin/clang++ -o "./build/MacOSX-10.10-...]
/Library/RubyMotion/lib/motion/project/builder.rb:320:in `build'
/Library/RubyMotion/lib/motion/project/app.rb:78:in `build'
/Library/RubyMotion/lib/motion/project/template/osx.rb:41:in `block (2 levels) in <top (required)>'
/Library/RubyMotion/lib/motion/project/template/osx.rb:59:in `block in <top (required)>'
Tasks: TOP => build:development
(See full trace by running task with --trace)
```

This is workaround for http://hipbyte.myjetbrains.com/youtrack/issue/RM-648