README.md: Update to require go 13 or newer
README.md: Update to require go 13 or newer

When building with go 1.12 you will receive the following error:

internal/gitlab/gitlab.go:65:4: unknown field 'ForceAttemptHTTP2' in struct literal of type http.Transport
internal/gitlab/gitlab.go:100:4: unknown field 'ForceAttemptHTTP2' in struct literal of type http.Transport
note: module requires Go 1.13
Makefile:5: recipe for target 'build' failed
make: *** [build] Error 2

Signed-off-by: Jason Wessel <3f19c87103ac4cd0301ffd05a2421b687c7b3c77@windriver.com>

zaquestion/lab,zaquestion/lab