Support building with iOS 6 as deployment target

Change-Id: I050a5a5beeae594a5b5a72b76ea9d5094d18eb06
Reviewed-on: https://gerrit.spotify.net/gerrit/101727
Reviewed-by: William Sackfield <sackfield@spotify.com>
Merge pull request #8 from pereckerdal/ios6-compat

Support building with iOS 6 as deployment target