fix broken dependency (sqlite3 and active_record)

active_record 4.x requires sqlite3-ruby 1.x.

```sh
$ bundle install
$ bundle exec rspec spec
---
Using sqlite3 to run the tests.
/usr/local/opt/rbenv/versions/2.0.0-p481/lib/ruby/gems/2.0.0/gems/activerecord-4.0.12/lib/active_record/connection_adapters/connection_specification.rb:58:in `rescue in resolve_hash_connection': Specified 'sqlite3' for database adapter, but the gem is not loaded. Add `gem 'sqlite3'` to your Gemfile. (Gem::LoadError)
---
```