Travis still running OOM, let's try one per CPU.
Travis still running OOM, let's try one per CPU.

Limit pre-commit-go to 20 concurrent goroutines.

R=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1587893002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go