Add etherpad client to requirements

requirements.txt

@@ -18,6 +18,7 @@ coverage==4.5.4
 curtsies==0.3.0
 dnspython==1.16.0
 enum-compat==0.0.3
+etherpad-lite==0.5
 eventlet==0.25.1
 feedgen==0.8.0
 Flask==1.1.1