Pin html5 to 0.3.15 since 0.3.16 seems to be buggy.

* Based on IRC reports by Inez couple weeks back, we found out that
  cardfight.wikia.com:Embodiment_of_Armor,_Bahr?oldid=1148775
  would crash with the 0.3.16 version of html5.

* So, for now, pin html5 library to the exact version.

Change-Id: Iab2cb99b4190f1a9a68eb67ea2dff86b690b38f7