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/proto/map_test.go

@@ -2,12 +2,36 @@ package proto_test
 
 import (
 	"fmt"
+	"reflect"
 	"testing"
 
 	"github.com/golang/protobuf/proto"
 	ppb "github.com/golang/protobuf/proto/proto3_proto"
 )
 
+func TestMap(t *testing.T) {
+	var b []byte
+	fmt.Sscanf("a2010c0a044b657931120456616c31a201130a044b657932120556616c3261120456616c32a201240a044b6579330d05000000120556616c33621a0556616c3361120456616c331505000000a20100a201260a044b657934130a07536f6d6555524c1209536f6d655469746c651a08536e69707065743114", "%x", &b)
+
+	var m ppb.Message
+	if err := proto.Unmarshal(b, &m); err != nil {
+		t.Fatalf("proto.Unmarshal error: %v", err)
+	}
+
+	got := m.StringMap
+	want := map[string]string{
+		"":     "",
+		"Key1": "Val1",
+		"Key2": "Val2",
+		"Key3": "Val3",
+		"Key4": "",
+	}
+
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("maps differ:\ngot  %#v\nwant %#v", got, want)
+	}
+}
+
 func marshalled() []byte {
 	m := &ppb.IntMaps{}
 	for i := 0; i < 1000; i++ {