Add -U to pip install command in tox.ini

Ask pip to upgrade packages in the virtualenv so that a long-lived
virtualenv is still kept up to date as external dependencies change.

Change-Id: Ieae9d6f642d8ef93c699d434698c53fc1fa4e3eb

tox.ini

@@ -6,7 +6,7 @@ skipsdist = True
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-install_command = pip install {opts} {packages}
+install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        setuptools_git>=0.4