internal/proxy: port script from Go Cloud to create module proxy (#95)

I have created gs://wire-modules to store the Wire dependencies. I ran
the script in this commit to populate the contents of the GCS bucket and
am updating Travis to point to the new proxy.

Fixes #81

internal/proxy/makeproxy.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+# Copyright 2018 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.
+
+# This script downloads and arranges packages in a structure for a
+# Wire proxy.
+
+# https://coderwall.com/p/fkfaqq/safer-bash-scripts-with-set-euxo-pipefail
+set -euxo pipefail
+
+if [[ $# -ne 1 ]]; then
+  echo "usage: makeproxy.sh <dir>" 1>&2
+  exit 64
+fi
+
+export GOPATH="$1"
+root="$( cd "$( dirname "${BASH_SOURCE[0]}" )/../.." >/dev/null && pwd )"
+cd "$root"
+go mod download