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/golang/protobuf/regenerate.sh

@@ -0,0 +1,53 @@
+#!/bin/bash
+
+set -e
+
+# Install the working tree's protoc-gen-gen in a tempdir.
+tmpdir=$(mktemp -d -t regen-wkt.XXXXXX)
+trap 'rm -rf $tmpdir' EXIT
+mkdir -p $tmpdir/bin
+PATH=$tmpdir/bin:$PATH
+GOBIN=$tmpdir/bin go install ./protoc-gen-go
+
+# Public imports require at least Go 1.9.
+supportTypeAliases=""
+if go list -f '{{context.ReleaseTags}}' runtime | grep -q go1.9; then
+  supportTypeAliases=1
+fi
+
+# Generate various test protos.
+PROTO_DIRS=(
+  conformance/internal/conformance_proto
+  jsonpb/jsonpb_test_proto
+  proto
+  protoc-gen-go/testdata
+)
+for dir in ${PROTO_DIRS[@]}; do
+  for p in `find $dir -name "*.proto"`; do
+    if [[ $p == */import_public/* && ! $supportTypeAliases ]]; then
+      echo "# $p (skipped)"
+      continue;
+    fi
+    echo "# $p"
+    protoc -I$dir --go_out=plugins=grpc,paths=source_relative:$dir $p
+  done
+done
+
+# Deriving the location of the source protos from the path to the
+# protoc binary may be a bit odd, but this is what protoc itself does.
+PROTO_INCLUDE=$(dirname $(dirname $(which protoc)))/include
+
+# Well-known types.
+WKT_PROTOS=(any duration empty struct timestamp wrappers)
+for p in ${WKT_PROTOS[@]}; do
+  echo "# google/protobuf/$p.proto"
+  protoc --go_out=paths=source_relative:$tmpdir google/protobuf/$p.proto
+  cp $tmpdir/google/protobuf/$p.pb.go ptypes/$p
+  cp $PROTO_INCLUDE/google/protobuf/$p.proto ptypes/$p
+done
+
+# descriptor.proto.
+echo "# google/protobuf/descriptor.proto"
+protoc --go_out=paths=source_relative:$tmpdir google/protobuf/descriptor.proto
+cp $tmpdir/google/protobuf/descriptor.pb.go protoc-gen-go/descriptor
+cp $PROTO_INCLUDE/google/protobuf/descriptor.proto protoc-gen-go/descriptor