Removing usage of the obsolete xulrunner binary

--- usr/bin/pencil

+++ usr/bin/pencil

@@ -1,5 +1,15 @@

 #!/bin/sh

 #

-/usr/bin/xulrunner --app "/usr/share/pencil/application.ini"

+DIR=/usr/share/pencil/

+if which firefox 2>/dev/null ; then

+    firefox -app $DIR/application.ini "$@";

+elif which iceweasel 2>/dev/null ; then

+    iceweasel -app $DIR/application.ini "$@";

+elif which abrowser 2>/dev/null ; then

+    abrowser -app $DIR/application.ini "$@";

+else

+    echo "Could not find XULRunner runtime engine."

+    exit 1;

+fi