travis, Dockerfile, appveyor, build: bump to Go 1.11

[SamuelMarks]

Release notes: https://golang.org/doc/go1.11

Don't merge until Go 1.11 appears in https://launchpad.net/~gophers/+archive/ubuntu/archive/+packages and https://hub.docker.com/_/golang/ (docker-library/golang#234)

[karalabe]

Please bump the docker images too, the general travis build steps too, and add 1.10 as a legacy step.

[SamuelMarks]

Looks like the new Docker images aren't available yet: https://hub.docker.com/_/golang/

[nonsense]

@SamuelMarks I believe we have to update more files than that. See previous PRs that added Go 1.10 as an example: #16128

[karalabe]

You are still missing the 1.10.x Travis build steps.

[SamuelMarks]

😅 need to improve my sed foo!

[karalabe]

Thanks for this! I've opened a superseding PR (with your commit cherry picked) as we need a few more code changes to make everything compatible with Go 1.11. I also needed to make a tiny fix to your PR: the ubuntu xenial packages cannot be upgraded to Go 1.11 because it has been end-of-life-d before 1.11 appeared, so there are no 1.11 packages.

That said, there's no point to update those because we're dropping support for all the images in the containers folder in favor of the main alpine images.

Updated PR at: #17701