Make 1.11 the explicitly named case, and 1.12 the default.

Change-Id: Ic3674860eb9e8a5bb1cf1da12c5ac583cc0b0633
Reviewed-on: https://go-review.googlesource.com/c/155397Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>