Update moment-recur.js - fixing IE9/10 toString.call() bug

changed the toString.call(units) calls in unitsToObject() function - lines 292 & 300 to Object.prototype.toString.call(units) because IE9 & IE10 were throwing "SCRIPT65535: Invalid calling object" errors. Tested in IE9/10 FF 27.0.1 & Chrome 33.0.1750.154 m