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: I5a57debd5c4bc03e6d00d3801be614d9b895c508

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install {opts} {packages}
+install_command = pip install -U {opts} {packages}
 setenv = LANG=en_US.UTF-8
          LANGUAGE=en_US:en
          LC_ALL=C