build: Bump MACOSX_DEPLOYMENT_TARGET to 10.7
Ref: #5731 (closed).
We currently target 10.5. Libuv is going to drop 10.6 in v2: libuv/libuv#758.
Note that OS X versions 10.5-10.8 are not supported by Apple anymore and do not receive security patches.
Note that we don't state supported OS X versions anywhere, and the only version tested at CI is 10.10. Ref: https://github.com/nodejs/build/issues/367.
/cc @bnoordhuis, @ofrobots, @jbergstroem, @nodejs/ctc