CRM-13243 - Don't use convertCheckboxes with SELECT[name=is_enrolled]

The SELECT works with the default converter but not the checkbox-converter.
We'll require opt-in for checkboxConverter rather than automatically using
it on all booleans.
Merge pull request #121 from civicrm/master-is_enrolled

CRM-13243 - Don't use convertCheckboxes with SELECT[name=is_enrolled]