Update zipzap.podspec

Update version from 6.0 to 6.1.
Release version (tags) are mapped to a certain commit.
6.0 is mapped to 7fe656be30e550b97258579a6763a09c59bfd660

Cocoapods clones that commit in stead of c27f9e101eb545b464c16595fbac7c326203cc41 - the one that has the latest podspec and the AES encryption support.

Updating the podspec version to 6.1 and creating a new release (6.1) would allow CocoaPods to clone the correct version.