all: add dependency checking to the script Travis runs (#128)

Dependency checking for CI - ensures a PR doesn't modify our
dependencies without anyone noticing.

Similar to https://github.com/google/go-cloud/pull/1284

Updates #127

internal/listdeps.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+# Copyright 2019 The Wire Authors
+#
+# 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
+#
+#     https://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.
+
+set -euo pipefail
+
+# To run this script manually to update alldeps:
+#
+# $ internal/listdeps.sh > internal/alldeps
+#
+# Important note: there are changes in module tooling behavior between go 1.11
+# and go 1.12; please make sure to use the same version of Go as used by Travis
+# (see .travis.yml) when updating the alldeps file.
+go list -deps -f '{{with .Module}}{{.Path}}{{end}}' ./... | sort | uniq