[CocoaPods/CocoaPods #1546] Change behavior of copy_resources_script to ...

...prevent compiling unneeded .xcasset files that are not part of the current target

Fixes issue #1546 which was causing the wrong xcasset files to get compiled in certain scenarios.
Please leave the double quotes around the xcrun. Without them, this will fail if Xcode is installed in a directory with a space. Which I introduced in 01907abf8dfa57312c0e7dd2b63dc7477fe5ba77.

```bash
$ mv /Applications/Xcode-Beta.app /Applications/Xcode\ Beta.app
$ sudo xcode-select --switch /Applications/Xcode\ Beta.app
$ [ `xcrun --find actool` ]
bash: [: /Applications/Xcode: unary operator expected
$ [ "`xcrun --find actool`" ]
```