Bump eventmachine to 1.0.3

When installing nats on clean environment currently it breaks with error:

```
$ gem install nats
ERROR:  While executing gem ... (Gem::DependencyError)
    Unable to resolve dependencies: nats requires eventmachine (= 0.12.10); thin requires eventmachine (>= 1.0.0)
```

This was caused by new version of thin release that changed dependency on eventmachine.