utils/s3.py throws UnicodeDecodeError when attempting to download tarball from S3

While trying to retrieve a tarball from S3 and extract it locally using archive.tar, I ran into this error below.

    Traceback (most recent call last):
      File "/root/salt/salt/states/file.py", line 1284, in managed
        makedirs
      File "/root/salt/salt/modules/file.py", line 2798, in manage_file
        sfn = __salt__['cp.cache_file'](source, saltenv)
      File "/root/salt/salt/modules/cp.py", line 314, in cache_file
        result = __context__['cp.fileclient'].cache_file(path, saltenv)
      File "/root/salt/salt/fileclient.py", line 145, in cache_file
        return self.get_url(path, '', True, saltenv)
      File "/root/salt/salt/fileclient.py", line 542, in get_url
        raise MinionError('Could not fetch from {0}'.format(url))
    MinionError: Could not fetch from s3://my.fancy.bucket/test-build-123.tar.gz

I traced the issue to [line 192 in utils/s3.py](https://github.com/saltstack/salt/blob/develop/salt/utils/s3.py#L192):

    # This can be used to save a binary object to disk
    if local_file and method == 'GET':
        log.debug('Saving to local file: {0}'.format(local_file))
        with salt.utils.fopen(local_file, 'w') as out:
            out.write(response)
        return 'Saved to local file: {0}'.format(local_file)

Which throws this error:

    'ascii' codec can't encode character u'\u2039' in position 1: ordinal not in range(128)

The response object is getting decoded by the requests library. According to the [Requests docs](http://docs.python-requests.org/en/latest/user/quickstart/#response-content), for a binary object (like what we would expect to get/retrieve from S3), the _content_ property will not attempt to decode the returned object. This PR fixes the issue for me, though I am not sure if there are any unintended side effects.

For the record, here is the state I am using to generate the issue:

    generated_hsd[full_local_path] = {
      'archive': [
        'extracted',
        { 'name': app['deploy']['path'] },
        { 'user': 'myuser' },
        { 'group': 'mygroup' },
        { 'mode': 400 },
        { 'source': 's3://{0}/{1}'.format(bucket, build_archive) },
        { 'source_hash': 'md5={0}'.format(build_hash) },
        { 'tar_options': 'xf' },
        { 'archive_format': 'tar' },
        { 'if_missing': '{0}/{1}'.format(local_path, build) },
        { 'require': [
          { 'user': 'myuser' },
          { 'file': local_path }
        ]
        }
      ]
    }