Fix require import needed by Ember

Author: lxcodes

src/core.js

@@ -1,6 +1,7 @@
 import Axis from './axis';
 import CLASS from './class';
 import { isValue, isFunction, isString, isUndefined, isDefined, ceil10, asHalfPixel, diffDomain, isEmpty, notEmpty, getOption, hasValue, sanitise, getPathBox } from './util';
+import require from 'require';
 
 export var c3 = { version: "0.5.4" };