title: Modify dependency files using prepare_command
body: <p>I have a Pod that depends on <code>Mantle</code>. I want to use <code>prepare_command</code> in my podspec to modify a <code>Mantle</code> file. But, it seems like the command runs before the <code>Mantle</code> dependency is downloaded during <code>pod install</code>. Is there a way to modify the dependency files from within my podspec?</p>