Add instructions for go.mod support and update version

[Randall O'Reilly]

To build using go modules on, you need a go.mod file where you're building, which has the latest version of gopy as a requirement. This fixes for example the issue from #228 and perhaps some others.

$ go mod init
$ go get -u github.com/go-python/gopy@master

if you don't do this, you'll get some errors about not finding gopyh. stuff, e.g.:

./leabra.go:207:2: undefined: gopyh.DecRef
./leabra.go:213:2: undefined: gopyh.IncRef
./leabra.go:219:9: undefined: gopyh.NumHandles

also should update a new tag so maybe it will default to the right thing. will add this info to the README first though.

[Randall O'Reilly]

done finally