Explicitly install readline from homebrew.

Addresses the following error:

```
Error: Parameter ensure failed on Package[readline]: Provider must have features 'upgradeable' to set 'ensure' to 'latest' at /opt/boxen/repo/shared/python/manifests/init.pp:19
Wrapped exception:
Provider must have features 'upgradeable' to set 'ensure' to 'latest'
Wrapped exception:
Provider must have features 'upgradeable' to set 'ensure' to 'latest'
```