Callback used for `unhandledrejection` event

@param {PromiseRejectionEvent} event An object containing
promise: the Promise that was rejected
reason: the value with which the Promise was rejected
@return void
function _promiseRejectionHandler(event) {
	    this._logDebug('debug', 'Raven caught unhandled promise rejection:', event);

	    this.captureException(event.reason, {
	      mechanism: {
	        type: 'onunhandledrejection',
	        handled: false
	      }
	    });
	  }