build: Bump MACOSX_DEPLOYMENT_TARGET to 10.7

Refs: https://github.com/nodejs/build/issues/367

libuv is going to drop 10.6 in v2.
Ref: https://github.com/libuv/libuv/pull/758

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

PR-URL: https://github.com/nodejs/node/pull/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>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>