Roll version of gsutil to 4.15.

third_party/gsutil/gslib/addlhelp/crc32c.py

Index: third_party/gsutil/gslib/addlhelp/crc32c.py
diff --git a/third_party/gsutil/gslib/addlhelp/crc32c.py b/third_party/gsutil/gslib/addlhelp/crc32c.py
index d2befe2e39f7fec48c109ede4074929ab198d42e..ed2bd8d4aa36a873972566b877bd2173f035c08a 100644
--- a/third_party/gsutil/gslib/addlhelp/crc32c.py
+++ b/third_party/gsutil/gslib/addlhelp/crc32c.py
@@ -110,7 +110,19 @@ _DETAILED_HELP_TEXT = ("""
   https://pypi.python.org/pypi/crcmod/1.7
 
   MSI installers are available for the 32-bit versions of Python 2.6 and 2.7.
+  Make sure to install to a 32-bit Python directory. If you're using 64-bit
+  Python it won't work with 32-bit crcmod, and instead you'll need to install
+  32-bit Python in order to use crcmod.
 
+  Note: If you have installed crcmod and gsutil hasn't detected it, it may have
+  been installed to the wrong directory. It should be located at
+  <python_dir>\\files\\Lib\\site-packages\\crcmod\\
+
+  In some cases the installer will incorrectly install to
+  <python_dir>\\Lib\\site-packages\\crcmod\\
+
+  Manually copying the crcmod directory to the correct location should resolve
+  the issue.
 """)