bump minimum OS X version to 10.7

[thestinger]

Rust is currently broken on 10.6 due to #10842, and there does not appear to be interest in fixing it. The version in the README should be increased if it's not going to be supported anymore.

Apple does not appear to support Snow Leopard anymore and I don't see a reason to continue support for an OS without security updates.

[thestinger]

cc @brson

[brson]

I'm ok with it. If somebody needs 0.6 support it still isn't that hard to add back. Anybody else have opinions? @pcwalton?

[alexcrichton]

I'm fine with bumping to 10.7, but I also haven't had a 10.6 machine in ages so I may be biased.

[brson]

I do have a 10.6 machine, but I haven't used it since Rust has been broken.