use es5's isNaN instead of es6's Number.isNaN

Trying to get this to work in IE. Number.isNaN [isn't available](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN#Browser_compatibility). No surprise here! :pensive: 


I thought about using a [polyfill](https://github.com/dannyfritz/pretty-bytes/compare/sindresorhus:master...master#diff-49257e95325752045a8c08b8a44c66d4L12) for Number.isNaN, but extending the native Number Object for this module seems like massive overkill and just bad practice overall.

There are some [quirks about isNaN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN#Confusing_special-case_behavior) that Number.isNaN was created to avoid. Checking with `typeof` before using isNaN should cover the edge cases.