Redefine google to google_public in common.h

There is a runtime exception when using google namespace on iPhone 5s
and iPad Air since Apple has its own copy of protobuf. See
http://stackoverflow.com/questions/19848118/weird-ios-libprotobuf-dylib-
cause-crash/20586291#20586291 for more info.
Merge pull request #6250 from jlansing/master

Redefine google to google_public in common.h