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/conformance/test.sh

@@ -0,0 +1,26 @@
+#!/bin/bash
+
+PROTOBUF_ROOT=$1
+CONFORMANCE_ROOT=$1/conformance
+CONFORMANCE_TEST_RUNNER=$CONFORMANCE_ROOT/conformance-test-runner
+
+cd $(dirname $0)
+
+if [[ $PROTOBUF_ROOT == "" ]]; then
+  echo "usage: test.sh <protobuf-root>" >/dev/stderr
+  exit 1
+fi
+
+if [[ ! -x $CONFORMANCE_TEST_RUNNER ]]; then
+  echo "SKIP: conformance test runner not installed" >/dev/stderr
+  exit 0
+fi
+
+a=$CONFORMANCE_ROOT/conformance.proto
+b=internal/conformance_proto/conformance.proto
+if [[ $(diff $a $b) != "" ]]; then
+  cp $a $b
+  echo "WARNING: conformance.proto is out of date" >/dev/stderr
+fi
+
+$CONFORMANCE_TEST_RUNNER --failure_list failure_list_go.txt ./conformance.sh