[gsutil.vpython] Update packages.

Update "google-compute-engine" and "boto" package versions.

Newer "gsutil" versions expect a newer "google-compute-engine" package
version, and fail with an ImportError on "constants.py".

BUG=None
TEST=prod
  - ssh into failing bot, ran with current "gsutil.vpython", reproduced.
  - ssh into failing bot, ran with new "gsutil.vpython", fixed.
  - ssh into old GCE image, ran with new "gsutil.vpython", works.

Change-Id: Ied277b6dc1cda4bc46cc9d58a633620c055e9d7c
Reviewed-on: https://chromium-review.googlesource.com/706807
Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>

gsutil.vpython

@@ -13,11 +13,11 @@
 
 wheel <
   name: "infra/python/wheels/google_compute_engine-py2_py3"
-  version: "version:2.3.7"
+  version: "version:2.6.2"
 >
 wheel <
   name: "infra/python/wheels/boto-py2_py3"
-  version: "version:2.47.0"
+  version: "version:2.48.0"
 >
 
 # "gsutil" on non-GCE can require PyOpenSSL, which, in turn, requires