meteor/startup.js reload update

re #17 

`Reload._onMigrate` added as a "newer" method of registering callback on hot code reload

`Meteor._reload.onMigrate` is deprecated (but should still be supported)

https://github.com/meteor/meteor/blob/d477c8d03bb078f7e8e85dbe4b51db7ae5689573/packages/reload/deprecated.js

(also flipped the logic to fail faster, so the function is easier to read)