This is to reflect the UTC decision to change the encoding model of
New Tai Lue from logical to visual to be similar to Thai, Lao, and
Tai Viet: http://www.unicode.org/L2/L2014/14250.htm#141-C26

The visual encoding is already the current practice of encoding New
Tai Lue on the web anyway:
http://www.unicode.org/L2/L2014/14195-newtailue.txt

Fixes behdad/harfbuzz#66.