Six import error in mock

.travis/install.sh

@@ -10,9 +10,9 @@ fi
 
 if python --version 2>&1 | grep -q PyPy; then
   # cryptography requires PyPy >= 2.6, Travis CI uses 2.5.0
-  $SCAPY_SUDO pip install $PIP_INSTALL_FLAGS mock
+  $SCAPY_SUDO pip install $PIP_INSTALL_FLAGS -U mock
 else
-  $SCAPY_SUDO pip install $PIP_INSTALL_FLAGS cryptography mock
+  $SCAPY_SUDO pip install $PIP_INSTALL_FLAGS -U cryptography mock
 fi
 
 # Install coverage