Add `go 1.11` to go.mod.
Recent versions of go (at least go1.14) automatically add a `go` line to
go.mod with every command, such as `go build`. Adding one here so that
builds don't wind up changing a versioned file. There doesn't seem to be
much importance to what number goes here
(https://utcc.utoronto.ca/~cks/space/blog/programming/GoModulesGoVersions).
I've just verified that the programs build with go1.11.
David Fifield