Update to Omnibus 4

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

The `mkdir` is not needed. Try:

```ruby
env = with_standard_compiler_flags(with_embedded_path)

command "berks install" \
        " --path=#{install_dir}/embedded/cookbooks", env: env, cwd: "#{project_dir}/opscode-pushy-server"
```