Update to Omnibus 4

:construction: 

Update push server project to Omnibus 4.  
- [ ] Clean build in CI.

Ready for review @opscode/release-engineers @manderson26 

I think we need to just pick a side on the `bundle exec` vs `binstub` fight. For consistency let's just recommend `bundle exec`:

```
$ kitchen login ubuntu-1204
[vagrant@ubuntu...] $ cd omnibus-pushy
[vagrant@ubuntu...] $ bundle install
[vagrant@ubuntu...] $ ...
[vagrant@ubuntu...] $ bundle exec omnibus build opscode-push-jobs-server -l internal
```