build: Bump MACOSX_DEPLOYMENT_TARGET to 10.7

Ref: nodejs/build#367

libuv is going to drop 10.6 in v2.
Ref: libuv/libuv#758

OS X versions below 10.9 are not supported by Apple anymore and do not
receive security patches.

PR-URL: #6402
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

common.gypi

@@ -308,7 +308,7 @@
           'GCC_ENABLE_PASCAL_STRINGS': 'NO',        # No -mpascal-strings
           'GCC_THREADSAFE_STATICS': 'NO',           # -fno-threadsafe-statics
           'PREBINDING': 'NO',                       # No -Wl,-prebind
-          'MACOSX_DEPLOYMENT_TARGET': '10.5',       # -mmacosx-version-min=10.5
+          'MACOSX_DEPLOYMENT_TARGET': '10.7',       # -mmacosx-version-min=10.7
           'USE_HEADERMAP': 'NO',
           'OTHER_CFLAGS': [
             '-fno-strict-aliasing',