Update to new cffi wheel.

This removes the libffi.so.6 dependency on Linux.

Bug: 1247674
Change-Id: I271bc6525ffbe8bd413e730b23365e1bc52aa16f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3299731
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Brian Ryner <bryner@google.com>

gsutil.vpython

@@ -51,7 +51,7 @@ wheel: <
 
 wheel: <
   name: "infra/python/wheels/cffi/${vpython_platform}"
-  version: "version:1.14.5.chromium.3"
+  version: "version:1.14.5.chromium.6"
 >
 
 wheel: <