Downgrade tests to ES5 syntax

The test suite for can-ajax threw syntax errors on IE11 due to the presence of ES6 arrow functions. StealJS does not automatically transpile to ES5 when using the CommonJS format as all CanJS sources do, so these function were being passed to the script engine unmodified.

This patch release does not involve changes to the can-ajax production code, and is primarily to ensure that the CanJS test suite runs smoothly on IE11.