Remove lsbdistcodename conditional for rbenv
Remove lsbdistcodename conditional for rbenv

All machines which include this class are now running trusty:

```
fab production puppet_class:govuk_rbenv::all num:1 do:'cat /etc/issue'
```

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet