lock.sh: suppress error output when looking for flock

Fixes (on centos6):
which: no flock in (/home/linuxbrew/.linuxbrew/bin:/home/linuxbrew/.linuxbrew/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin)

Library/Homebrew/utils/lock.sh

@@ -45,7 +45,7 @@ _create_lock() {
   if [[ -x "$ruby" ]] && "$ruby" -e "exit(RUBY_VERSION >= '1.8.7')"
   then
     "$ruby" -e "File.new($lock_fd).flock(File::LOCK_EX | File::LOCK_NB) || exit(1)"
-  elif [[ -x "$(which flock)" ]]
+  elif [[ -x "$(which flock 2>/dev/null)" ]]
   then
     flock -n "$lock_fd"
   elif [[ -x "$python" ]]