Fixes deprecation warnings with formtastic

This fixes the depcrecation warnings with formtastic. I also updated
ActiveAdmin to v0.5.1. We will still need to remove
ActiveAdmin::Dashboard which is also depcrecated as of v0.6.0 (see #11
& #12).
Merge pull request #22 from 18F/feature/fix_formtastic_deprecation_warnings

Fixes deprecation warnings with formtastic