Use ensure_resource() from stdlib for the readline package in order to a...

Avoid a duplicate declaration when used with puppet-ruby.

```
> ./script/boxen
Error: Duplicate declaration: Package[readline] is already declared; cannot redeclare at /opt/boxen/repo/shared/python/manifests/init.pp:19 on node mymacbook
Error: Duplicate declaration: Package[readline] is already declared; cannot redeclare at /opt/boxen/repo/shared/python/manifests/init.pp:19 on node mymacbook
```