Do not rely on ~/.cocoapods being present.

Currently, CP fails a bit miserably when its dot-directory doesn't exist. This PR makes it work again if explicit sources are used. 

I am wondering though, if it should also do a `pod repo setup` automatically in case there are no explicit sources defined? I believe this was the behaviour before we deprecated implicit sources and since we undeprecated them, we should probably restore that behaviour as well.