Gate some emoji tests on a new enough iOS version.
Ubuntu 16.04 doesn't have a recent enough ICU to support these; we need a better long-term solution, such as bundling ICU with the toolchain.
- numericValue returns nil instead of .nan for non-numerics - Remove small-string optimizations from _scalarName that failed on 32-bit archs - Put case mappings back into U.S.Properties - Added more sanity tests
