Log Message:
Update to version 1.3.0.

Pkgsrc changes:
 * Update PLIST

Upstream changes:
 * #699: add a go.mod module file
 * #701: stop generating package "// import" comment
 * #741: deprecate {Unm,M}arshalMessageSet{JSON}
 * #760: different internal implementation of oneofs
 * `.pb.go` files generated by `protoc-gen-go@v1.3.0` will require
   the `proto@v1.3.0` package to work
 * various minor changes to code generation