Disallow gsutil installs on Python != 2.7.
Disallow gsutil installs on Python != 2.7.

Users currently can get all the way through the process of installing
gsutil on a system with Python 2.6 before hitting the runtime check that
tells them they can't use it without 2.7.  This prevents Pip from
installing it on Python versions other than 2.7.

GoogleCloudPlatform/gsutil,GoogleCloudPlatform/gsutil