Fix "cannot overwrite directory" when copying OS X frameworks into the resources dir

As mentioned in #1063, the following error can occur if the framework already exists in the target location:

> cannot overwrite directory /Users/nik/Library/Developer/Xcode/DerivedData/XXX-fblzblixtvaiirdhllllqolnnsbq/Build/Products/Debug/XXX.app/Contents/Frameworks/DropboxOSX.framework/Versions/Current with non-directory /Users/nik/src/XXX/Pods/Dropbox-OSX-SDK/dropbox-osx-sdk-1.3.5/DropboxOSX.framework/Versions/Current

The change was briefly discussed in #1063 and nobody objected to it.