feat(watcher): use polling on Mac

Revert to polling on Mac. Use OS events on Linux and Windows.

Eventually I wanna get rid off polling even on Mac, however it's not ready yet.

See:
https://github.com/pipobscure/fsevents/issues/17
paulmillr/chokidar#87

Basically reverts e88fbc2.

lib/config.js

@@ -196,7 +196,7 @@ var Config = function() {
   this.colors = true;
   this.autoWatch = true;
   this.autoWatchBatchDelay = 250;
-  this.usePolling = false;
+  this.usePolling = process.platform === 'darwin';
   this.reporters = ['progress'];
   this.singleRun = false;
   this.browsers = [];