Fix replaceValue for currency symbol replacements

This is an alternate approach to https://github.com/cpettitt/Numeral-js/commit/29bad4e6ff4a44de00ed51cbec0a62d9173b9041 which did not work with older non ECMAScript browsers (IE 7, 8, 9).

I've tested this in the latest Chrome, Firefox, Safari, Nashorn (via ad-hoc testing), Node, and IE 7/8/9/10/11.