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: I67288f12bc125c12d124cd1dcfeff5ec70975d48

tox.ini

@@ -17,7 +17,7 @@ deps =
 whitelist_externals = bash
                       find
                       rm
-install_command = pip install -U {opts} {packages}
+install_command = pip install {opts} {packages}
 commands =
   find . -type f -name "*.pyc" -delete
   rm -Rf .testrepository/times.dbm