Change default gsutil.py version to 4.13

BUG=
TESTS=
alpaca(gsutil_version) ~/depot_tools$ ./gsutil.py version                                                                                                                                                   
gsutil version: 4.13

Review URL: https://codereview.chromium.org/1237013003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296023 0039d316-1c4b-4281-b951-d872f2087c98

gsutil.py

@@ -108,7 +108,7 @@ def run_gsutil(force_version, fallback, target, args):
 
 def parse_args():
   parser = argparse.ArgumentParser()
-  parser.add_argument('--force-version')
+  parser.add_argument('--force-version', default='4.13')
   parser.add_argument('--fallback', default=DEFAULT_FALLBACK_GSUTIL)
   parser.add_argument('--target', default=DEFAULT_BIN_DIR)
   parser.add_argument('args', nargs=argparse.REMAINDER)