Add -U flag to pip install command

The -U flag makes tox able to upgrade all packages to the newest available
version.

Change-Id: Ibe5fa0115c2b151ae6f3bd741cc561d2435499e8

tox.ini

@@ -5,7 +5,7 @@ envlist = pep8
 
 [testenv]
 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
 setenv = VIRTUAL_ENV={envdir}