Fix gemspec

[rstacruz]

The current gemspec fails to build:

ERROR:  While executing gem ... (Gem::InvalidSpecificationException)
    duplicate dependency on activeadmin (~> 0.4.3, development), (>= 0.4.0) use:
    add_runtime_dependency 'activeadmin', '~> 0.4.3', '>= 0.4.0'

This fixes that. This also allows you to use this gem via GitHub:

gem 'active_admin_editor', github: 'ejholmes/active_admin_editor'

[rstacruz]

Also, I'm offering help to clean up the gem and release it, just let me know.

[ruprict]

@rstacruz I would say, just do it. Release it from your fork if needed. Clearly, @ejholmes has moved on from this gem.