The download page says "OS X 10.8 or later", but other pages said 10.7.
Say 10.8 everywhere.

Turns out Go doesn't even compile on OS X 10.7 (details in bug) and we
only run builders for OS X 10.8+, which is likely why 10.7
regressed. Until recently we only had OS X 10.10 builders, even.

We could run 10.7 builders, but there's basically no reason to do so,
especially with 10.12 coming out imminently.

Fixes #16625

Change-Id: Ida6e20fb6c54aea0a3757235b708ac1c053b8c04
Reviewed-on: https://go-review.googlesource.com/28870Reviewed-by: Chris Broadfoot <cbro@golang.org>