install-on-debian: install python-lxml

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
diff --git a/install-on-debian.sh b/install-on-debian.sh
index e203ffa..8a9d80d 100755
--- a/install-on-debian.sh
+++ b/install-on-debian.sh
@@ -6,6 +6,7 @@
 prerequirements () {
     sudo apt-get install -y python2.7
     sudo apt-get install -y python-pip
+    sudo apt-get install -y python-lxml
     sudo pip install robotframework
     sudo pip install robotframework-selenium2library
 }