Replace deprecated String.prototype.substr()

[CommanderRoot]

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated. .substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

[taylorhakes]

Hi @CommanderRoot. Thanks for the suggestion about substr. I know MDN says it's deprecated, but Javascript doesn't deprecate things, so it will never be removed and I prefer the API. I would prefer to keep it as is.