Using "let" requires ES6 since v0.10.2

[mherold]

Not sure whether this was intended, but during the fix of #84 you introduced a let (which is not available in ES5). This may break compatibility when doing a patch update from v0.10.1 to v0.10.2 (Unexpected token: name (year)).

https://github.com/samsonjs/strftime/blob/v0.10.2/strftime.js#L652