UnhandledPromiseRejectionWarning in completeQueueHandler

[cschwebk]

In the completeQueueHandler method of src/test_runner.js, an Unhandled Promise Rejection will be triggered if a reporter (such as magellan-xunit-reporter) does not return a promise out of the flush method. I suspect it will also be triggered if flush does not exist in the reporter. It looks like these checks were removed in #257.

(node:41618) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'then' of undefined
    at innerResolve (.../node_modules/testarmada-magellan/src/test_runner.js:196:21)
    at new Promise (<anonymous>)
    at Promise.all._.map.listener (.../node_modules/testarmada-magellan/src/test_runner.js:193:15)
    at arrayMap (.../node_modules/lodash/lodash.js:639:23)
    at Function.map (.../node_modules/lodash/lodash.js:9556:14)
    at Timeout.setTimeout [as _onTimeout] (.../node_modules/testarmada-magellan/src/test_runner.js:190:13)
    at ontimeout (timers.js:438:13)
    at tryOnTimeout (timers.js:300:5)
    at listOnTimeout (timers.js:263:5)
    at Timer.processTimers (timers.js:223:10)
(node:41618) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 2)
(node:41618) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.