The RawProtocol only handles strings

Since the JSONProtocol and PickleProtocol end up calling into libraries that can handle
any output that the mapper or reducer yields. For the RawProtocol since it was doing a
string.join, errors would occur if a non string was yielded. For example I had a reducer
that was yielding a string for the key and an int as the value.

The stack trace I was getting was

Traceback (most recent call last):
  File "package_mobile_usage.py", line 66, in <module>
    Job.run()
  File "/usr/local/lib/python2.7/dist-packages/mrjob/job.py", line 500, in run
    mr_job.execute()
  File "/usr/local/lib/python2.7/dist-packages/mrjob/job.py", line 515, in execute
    self.run_reducer(self.options.step_num)
  File "/usr/local/lib/python2.7/dist-packages/mrjob/job.py", line 620, in run_reducer
    write_line(out_key, out_value)
  File "/usr/local/lib/python2.7/dist-packages/mrjob/job.py", line 746, in write_line
    print >> self.stdout, write(key, value)
  File "/usr/local/lib/python2.7/dist-packages/mrjob/protocol.py", line 152, in write
    return '\t'.join(x for x in (key, value) if x is not None)
TypeError: sequence item 1: expected string, int found

I'm not sure if just calling str() is the best way to handle this so let me know what you think.