Expliclity require 'timeout' from the stdlib

This avoids getting the following error on some platforms:

NameError: uninitialized constant GitlabKeys::Timeout

CHANGELOG

+v1.9.6
+  - Explicitly require 'timeout' from the standard library
+
 v1.9.5
   - Put authorized_keys.lock in the same directory as authorized_keys
   - Use lock file when add new entries to authorized_keys

lib/gitlab_keys.rb

 require 'tempfile'
+require 'timeout'
 
 require_relative 'gitlab_config'
 require_relative 'gitlab_logger'