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/spf13/pflag/golangflag_test.go

@@ -36,4 +36,12 @@ func TestGoflags(t *testing.T) {
 	if getBool != true {
 		t.Fatalf("expected getBool=true but got getBool=%v", getBool)
 	}
+	if !f.Parsed() {
+		t.Fatal("f.Parsed() return false after f.Parse() called")
+	}
+
+	// in fact it is useless. because `go test` called flag.Parse()
+	if !goflag.CommandLine.Parsed() {
+		t.Fatal("goflag.CommandLine.Parsed() return false after f.Parse() called")
+	}
 }