Specify packages being required by Selenium

Co-authored-by: David Furey <david.furey@guardian.co.uk>

roles/browser-testing/tasks/main.yml

@@ -15,11 +15,11 @@
 - name: Enable Xvfb service
   service: name=xvfb enabled=yes
 
-- name: Install required libxrender1 package
+- name: Install libxrender1, required by Selenium
   apt: name=libxrender1 state=latest
 
-- name: Install required libxtst6 package
+- name: Install libxtst6, required by Selenium
   apt: name=libxtst6 state=latest
 
-- name: Install required libxi6 package
+- name: Install libxi6, required by Selenium
   apt: name=libxi6 state=latest