Remove -U from pip install

'pip install -U' ugrades specified packages, this is not necessary
since we use constraints, remove the parameter '-U' from the line.

With tools/tox_install.sh - which a previous change of mine removed -
the -U was not harmful, but with the current set up, it might cause
upgrades, so remove it.

Change-Id: I2b4bf1e1c970afea899521e3cab08a817a186782

tox.ini

@@ -3,7 +3,7 @@ minversion = 2.0
 envlist = py35,py27,pypy,pep8
 
 [testenv]
-install_command = pip install -U {opts} {packages}
+install_command = pip install {opts} {packages}
 whitelist_externals = find
                       rm
 deps =