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).
Merge pull request #2 from bbarkley/master

Fix replaceValue for currency symbol replacements