Fix \v being treated as v in IE < 9

IE < 9 treats '\v' as 'v' instead of a vertical tab ('\x0B').

Read more: http://mathiasbynens.be/notes/javascript-escapes

@mathiasbynens originally found this bug in my Lua parser.
Discussion on luaparse: http://goo.gl/nmoLS
Fix on luaparse: http://goo.gl/v7nxL
Uglify2 also does this: http://goo.gl/xibiK