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: I0256cf4cc3aa7675956762dcf056e2709d36813a

tox.ini

@@ -9,7 +9,7 @@ setenv = VIRTUAL_ENV={envdir}
          LANGUAGE=en_US:en
          LC_ALL=C
 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
 commands = python setup.py testr --testr-args='{posargs}'