Add callback to async fs.utimes

[FezVrasta]

Get rid of deprecation warning on Node 7

(node:49216) DeprecationWarning: Calling an asynchronous function without callback is deprecated.
    at makeCallback (fs.js:103:12)
    at Object.fs.utimes (fs.js:1127:14)
    at /Users/federicozivolo/Progetti/popper-js/node_modules/karma-rollup-preprocessor/lib/index.js:22:7
    at Timeout.later [as _onTimeout] (/Users/federicozivolo/Progetti/popper-js/node_modules/debounce/index.js:34:23)
    at ontimeout (timers.js:365:14)
    at tryOnTimeout (timers.js:237:5)
    at Timer.listOnTimeout (timers.js:207:5)

[jlmakes]

Thanks!