The protobuf 1.3.3 depends on Go interpreter of at least 1.12. Per the
Go version support policy (two releases before the current release of
1.14), this version is unsupported anyway. This commit drops support for
1.11 and expands the test matrix to cover 1.13 and 1.14.
* support go modules
- use Go 1.11 (with and without module support) in CI
- add go.mod and go.sum files
- with modules in Go 1.11, can't rely on GOPATH directory layout, so just copy test.proto into this repo
Richard Belleville