vendor: update all dependencies

* Update all dependencies
  * Remove all `[[constraint]]` from Gopkg.toml
  * Add in the minimum number of `[[override]]` to build
  * Remove go get of github.com/inconshreveable/mousetrap as it is vendored
  * Update docs with new policy on constraints

vendor/github.com/golang/protobuf/protoc-gen-go/testdata/main_test.go

@@ -36,11 +36,13 @@ package testdata
 import (
 	"testing"
 
-	mytestpb "./my_test"
+	importspb "github.com/golang/protobuf/protoc-gen-go/testdata/imports"
 	multipb "github.com/golang/protobuf/protoc-gen-go/testdata/multi"
+	mytestpb "github.com/golang/protobuf/protoc-gen-go/testdata/my_test"
 )
 
 func TestLink(t *testing.T) {
 	_ = &multipb.Multi1{}
 	_ = &mytestpb.Request{}
+	_ = &importspb.All{}
 }