Security release of Golang 1.12.8 and 1.11.13

[matt-e]

Fixes for CVE-2019-9512 and CVE-2019-9514 released as part of golang/go#33606.

Would be great to know when we can see those land in rules_go.

[jayconrod]

Support for new versions has merged to master (#2178) already.

rules_go point releases will be available later tonight.

[jayconrod]

Releases have been published and announced.