Run pip install in cord-tester prerequisites in upgrade mode

Change-Id: Ib907db13012e63b0906530a0163294e42299eefc
diff --git a/src/test/setup/prerequisites.sh b/src/test/setup/prerequisites.sh
index 5a0944b..f0bbbfa 100755
--- a/src/test/setup/prerequisites.sh
+++ b/src/test/setup/prerequisites.sh
@@ -96,11 +96,11 @@
     virtualenv $setup_path/venv
     echo "Installing cord-tester pip packages on the virtual environment"
     $setup_path/venv/bin/pip install -U distribute
-    $setup_path/venv/bin/pip install -r $setup_path/requirements.txt
+    $setup_path/venv/bin/pip install -U -r $setup_path/requirements.txt
 else
     echo "Installing cord-tester pip packages on the host"
     pip install -U distribute
-    pip install -r $setup_path/requirements.txt
+    pip install -U -r $setup_path/requirements.txt
 fi
 
 ( cd /tmp && git clone https://github.com/jpetazzo/pipework.git && cp -v pipework/pipework /usr/bin && rm -rf pipework )