Make ListenWatcher optional closes #128

By default spring is configured to watch_via [`listen`](https://github.com/guard/listen) is available spring will use that, alternatively spring can be configured to use `:listen`, `:polling`, or a`CustomClass`.

```ruby
Spring.watch_via = :polling
```
Merge pull request #135 from kylewelsby/master

Make ListenWatcher optional. Closes #128