Required Go version in README is wrong

[aa2811]

Bug Report

What did you do?

Tried to go_get promtool in the golang:1.11.5 container. According to the README, go 1.11+ should be sufficient to do this.

What did you expect to see?
Sucessful docker build

What did you see instead? Under which circumstances?

Step 2/14 : RUN GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go get -u github.com/prometheus/prometheus/cmd/promtool
 ---> Running in ebf3f42eb56d
# github.com/prometheus/prometheus/vendor/github.com/prometheus/tsdb/goversion
src/github.com/prometheus/prometheus/vendor/github.com/prometheus/tsdb/goversion/init.go:17:9: undefined: _SoftwareRequiresGOVERSION1_12
The command '/bin/sh -c GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go get -u github.com/prometheus/prometheus/cmd/promtool' returned a non-zero code: 2

Environment

golang:1.11.5 container.

This step worked in golang:1.12.3, and judging by this file which has recently been added, I think this is because you now need go 1.12+ to build from source, so the README needs updating.

[simonpasquier]

Do you want to submit the PR for this?

[cstyan]

fixed in #5471