Update dep info

- Add pruning settings to Gopkg.toml
- Update vendoring deps doc to point to dep installation instructions
  and to use dep instead of hack/dep-save.sh
- Remove hack/dep-save.sh

Signed-off-by: Andy Goldstein <andy.goldstein@gmail.com>

hack/dep-save.sh

@@ -1,21 +0,0 @@
-#!/bin/bash -e
-#
-# Copyright 2017 the Heptio Ark contributors.
-#
-# 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.
-
-dep ensure $@
-dep prune
-
-# remove files we don't want
-find vendor \( -name BUILD -o -name .travis.yml -o -name '*_test.go' \) -exec rm {} \;