compile OS X with --std=c++1y

because Travis uses Apple LLVM 6.0 and full c++14 support landed in 6.1
https://developer.apple.com/library/ios/documentation/DeveloperTools/Conceptual/WhatsNewXcode/WhatsNewXcode.pdf

binding.gyp

@@ -28,11 +28,11 @@
         ['OS=="mac"', {
           'xcode_settings': {
             'CLANG_CXX_LIBRARY': 'libc++',
-            'CLANG_CXX_LANGUAGE_STANDARD': 'c++14',
             'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
             'GCC_ENABLE_CPP_EXCEPTIONS': 'YES',
             'GCC_ENABLE_CPP_RTTI': 'YES',
             'OTHER_CPLUSPLUSFLAGS': [
+              '-std=c++1y',
               '-Werror',
               '-Wall',
               '-Wextra',