reject apps symlinked to a nonexistant file

I got the following error when running `chairs open`
```
/Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/pow.rb:99:in `mtime': No such file or directory @ rb_file_s_mtime - /Users/artsy/Library/Application Support/iPhone Simulator/7.0/Applications/C608265B-FA2D-4979-98C1-64A392045EA0/DDActionsService.app (Errno::ENOENT)
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/pow.rb:99:in `modified_at'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/musician.rb:206:in `block (2 levels) in get_app_folder'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/directory.rb:105:in `each'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/directory.rb:105:in `each'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/musician.rb:194:in `block in get_app_folder'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/directory.rb:105:in `each'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/pow/directory.rb:105:in `each'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/musician.rb:186:in `get_app_folder'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/musician.rb:34:in `open'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/lib/musician.rb:17:in `initialize'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/bin/chairs:8:in `new'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/gems/chairs-1.0.2/bin/chairs:8:in `<top (required)>'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/bin/chairs:23:in `load'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/bin/chairs:23:in `<main>'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `eval'
	from /Users/artsy/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `<main>'
```
Apparently the files some of my apps link to no longer exist -- I'm not sure why this happens, but when it does, `modified_at` throws an error. Now this can be avoided.