Update CocoaLibSpotify.podspec

The download link of the post-install script doesn't work anymore. I'm not a member of the Spotify Team but since I wanted to check out the pod I found out that the download link was broken. After looking the correct download link up at their homepage it works again.
Update CocoaLibSpotify.podspec

I'm sorry, but I've overseen to even update the 's.source_files' property.
Merge pull request #1448 from hohl/patch-1

Update CocoaLibSpotify.podspec of version 2.1.0