fix(watcher): prevent Node crash when Karma exits

`.close()` method returns a Promise now.

See fsevents/fsevents#314 (comment).

Fixes karma-runner#3511

lib/watcher.js

@@ -67,8 +67,7 @@ function watch (patterns, excludes, fileList, usePolling, emitter) {
     .on('error', log.debug.bind(log))
 
   emitter.on('exit', (done) => {
-    watcher.close()
-    done()
+    watcher.close().then(() => done())
   })
 
   return watcher