Use system libjs-excanvas

In Debian, there's system-wide libjs-excanvas. This commit makes
tribler to use excanvas.js from system instead of local one.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Revert libx11-6 back in debian/control Depends.
Merge pull request #861 from grandpaul/use-system-libjs-excanvas

Use system libjs-excanvas