Update go111 to 1.11.9.

go1.11.7 (released 2019/04/05) includes fixes to the runtime and the net
packages. See the Go 1.11.7 milestone on our issue tracker for details.

go1.11.8 (released 2019/04/08) was accidentally released without its intended
fix. It is identical to go1.11.7, except for its version number. The intended
fix is in go1.11.9.

go1.11.9 (released 2019/04/11) fixes an issue where using the prebuilt binary
releases on older versions of GNU/Linux led to failures when linking programs
that used cgo. Only Linux users who hit this issue need to update.

(bsiegert)