Support for Go 1.11 with GO111MODULES=on

[gnossen]

It seems that #8 introduced a hard dependency on Go 1.13. 1.12 is still supported by the Go language team. And while 1.11 is no longer supported, it is still critical for environments like appengine, which have not yet dropped support. I ran into this problem while trying to compile grpcurl with Go 1.11 (PR):

go test -race ./...
go build golang.org/x/sys/unix: module requires Go 1.12
go build github.com/cncf/udpa/go/udpa/annotations: module requires Go 1.13
go build github.com/envoyproxy/go-control-plane/envoy/annotations: module requires Go 1.12
go build github.com/envoyproxy/go-control-plane/envoy/type: module requires Go 1.12
FAIL	github.com/fullstorydev/grpcurl [build failed]
FAIL	github.com/fullstorydev/grpcurl/cmd/grpcurl [build failed]
Makefile:82: recipe for target 'test' failed

There seems to be a general problem with the Go tooling auto-inserting hard dependencies like this when none was intended by the author. Is that the case here? Or is there some feature that the library truly does depend on in Go 1.13+?

CC @kyessenov
CC @htuch