gsutil.py: bump default version to 4.28 from 4.13

Picks up about 2 years worth of changes.  Should be relatively risk-free since
we pin gsutil.py in almost all of our scripts.

Bug:
Change-Id: Iff1365ee6d6f31e03da8d8876b6ccd0b47a033c2
Reviewed-on: https://chromium-review.googlesource.com/801355Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>

gsutil.py

@@ -156,7 +156,7 @@ def parse_args():
   bin_dir = os.environ.get('DEPOT_TOOLS_GSUTIL_BIN_DIR', DEFAULT_BIN_DIR)
 
   parser = argparse.ArgumentParser()
-  parser.add_argument('--force-version', default='4.13')
+  parser.add_argument('--force-version', default='4.28')
   parser.add_argument('--clean', action='store_true',
       help='Clear any existing gsutil package, forcing a new download.')
   parser.add_argument('--fallback', default=DEFAULT_FALLBACK_GSUTIL)