Bump Gsutil version for download_from_google_storage to 4.15

BUG=

Review URL: https://codereview.chromium.org/1352043004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296750 0039d316-1c4b-4281-b951-d872f2087c98

download_from_google_storage.py

@@ -55,7 +55,7 @@ def GetNormalizedPlatform():
 class Gsutil(object):
   """Call gsutil with some predefined settings.  This is a convenience object,
   and is also immutable."""
-  def __init__(self, path, boto_path=None, timeout=None, version='4.7'):
+  def __init__(self, path, boto_path=None, timeout=None, version='4.15'):
     if not os.path.exists(path):
       raise FileNotFoundError('GSUtil not found in %s' % path)
     self.path = path