This way, DEPOT_TOOLS_PYTHON_275=1 will properly upgrade an installation
still running 2.6. Otherwise the user had to "del python.bat" to force
the upgrade.

Stop calling python to verify it works, this is slow.

Upgrade url to https.

R=iannucci@chromium.org
BUG=241769

Review URL: https://codereview.chromium.org/26901003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@228179 0039d316-1c4b-4281-b951-d872f2087c98