bugfix: watchForUnresponsiveInstruments process run on systems without ios configured

If you tried to run appium on a system without ios configured, it would fail because it would try to start the watchForUnresponsiveInstruments process. Added a check to the config file to prevent it being run is an `ios` key is not present.

I noticed that the .appiumconfig file was being read by `fs.readSync` every time it was accessed. Renaming it to .appiumconfig.json allows us to `require()` it, and `require()` automatically caches successive calls.