Fix #2071 replacing /bin/sh with bash
On Ubuntu /bin/sh is dash by default, this can cause
generate_descriptor_proto.sh fail to execute.
diff --git a/generate_descriptor_proto.sh b/generate_descriptor_proto.sh
index c170c83..668e6d1 100755
--- a/generate_descriptor_proto.sh
+++ b/generate_descriptor_proto.sh
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
# Run this script to regenerate descriptor.pb.{h,cc} after the protocol
# compiler changes. Since these files are compiled into the protocol compiler
