Fix \v being treated as v in IE < 9

What steps will reproduce the problem?
1. Using esprima on IE < 9.
2. Parsing a string literal containing \v

What is the expected output?
v

What do you see instead?
\v

I actually haven't tested this as the test suite doesn't work in IE. However the problem is documented here http://mathiasbynens.be/notes/javascript-escapes

Mathias Bynens 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