Update Go version requirement

We run tests on current + (current -1) (we also test on the latest Go source), which means that the current supported Go versions are Go 1.12 and 1.13.

Closes #6853

README.md

@@ -54,7 +54,7 @@ Use the [installation instructions in the Hugo documentation](https://gohugo.io/
 #### Prerequisite Tools
 
 * [Git](https://git-scm.com/)
-* [Go (at least Go 1.11)](https://golang.org/dl/)
+* [Go (we test it with the last 2 major versions)](https://golang.org/dl/)
 
 #### Fetch from GitHub