Updated ZXing ios sub spec for supporting the latest cocoapod version

This will fix the build error.
ZXing (2.1) issue with latest stable version (0.18.1)
#issue 1008
Added the preserve_paths for ios sub pod spec
Merge pull request #1987 from NSMutableString/master

Update ZXing (2.1) pod spec to solve iOS sub spec issue with latest stable version (0.18.1)