Change typescript pin to ~2.3.
Change typescript pin to ~2.3.

This should unblock CQ, but should also prevent us from upgrading to 2.4, which
has some stricter checks that find bugs in logdog.

R=dnj@chromium.org, vadimsh@chromium.org
BUG=737631

Review-Url: https://codereview.chromium.org/2958313002

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