Upgrade to coverage 3.7.1 and have it auto-build itself on first use.

PRESUBMIT.py

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Top-level presubmit script for buildbot.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts for
 details on the presubmit API built into gcl.
 """
 
@@ skipping 16 matching lines @@
       r'.*masters/.*/public_html/.*\.html$',
       r'.*masters/.*/public_html/.*\.css$',
   ]
   tests = []
   sys_path_backup = sys.path
   try:
     sys.path = [
         join('third_party'),
         join('third_party', 'buildbot_8_4p1'),
         join('third_party', 'buildbot_slave_8_4'),
-        join('third_party', 'coverage-3.6'),
+        join('third_party', 'coverage-3.7.1'),
         join('third_party', 'decorator_3_3_1'),
         join('third_party', 'jinja2'),
         join('third_party', 'mock-1.0.1'),
         join('third_party', 'sqlalchemy_0_7_1'),
         join('third_party', 'sqlalchemy_migrate_0_7_1'),
         join('third_party', 'tempita_0_5'),
         join('third_party', 'twisted_10_2'),
         join('scripts'),
         # Initially, a separate run was done for unit tests but now that
         # pylint is fetched in memory with setuptools, it seems it caches
@@ skipping 69 matching lines @@
 
 
 def CheckChangeOnUpload(input_api, output_api):
   output = CommonChecks(input_api, output_api)
   output.extend(BuildInternalCheck(output, input_api, output_api))
   return output
 
 
 def CheckChangeOnCommit(input_api, output_api):
   return CheckChangeOnUpload(input_api, output_api)