Update dependencies, enable pruning for vendor/

So, `dep` got an nice new feature to remove tests and non-go files from
`vendor/`, and this brings the size of the vendor directory from ~300MiB
down to ~20MiB. We don that now.

vendor/google.golang.org/api/internal/creds.go

@@ -1,42 +0,0 @@
-// Copyright 2017 Google Inc. All Rights Reserved.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package internal
-
-import (
-	"fmt"
-	"io/ioutil"
-
-	"golang.org/x/net/context"
-	"golang.org/x/oauth2/google"
-)
-
-// Creds returns credential information obtained from DialSettings, or if none, then
-// it returns default credential information.
-func Creds(ctx context.Context, ds *DialSettings) (*google.DefaultCredentials, error) {
-	if ds.Credentials != nil {
-		return ds.Credentials, nil
-	}
-	if ds.CredentialsFile != "" {
-		data, err := ioutil.ReadFile(ds.CredentialsFile)
-		if err != nil {
-			return nil, fmt.Errorf("cannot read credentials file: %v", err)
-		}
-		return google.CredentialsFromJSON(ctx, data, ds.Scopes...)
-	}
-	if ds.TokenSource != nil {
-		return &google.DefaultCredentials{TokenSource: ds.TokenSource}, nil
-	}
-	return google.FindDefaultCredentials(ctx, ds.Scopes...)
-}