Fix conflict with ActiveSupport dependency loading

`Object#require` in ActiveSupport attempts to call their own `Object#load_dependency` method [1], which is overridden in `Adapter` and causes an error when Lotus Model is used with Rails. This addresses the conflict by choosing a different method name.

```
/opt/boxen/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bundler/gems/model-17b037ea2cb5/lib/lotus/model/config/adapter.rb:85:in `load_dependency': wrong number of arguments (1 for 0) (ArgumentError)
        from /opt/boxen/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/activesupport-4.2.0.rc1/lib/active_support/dependencies.rb:274:in `require'
        from /opt/boxen/rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/bundler/gems/model-17b037ea2cb5/lib/lotus/model/config/adapter.rb:87:in `load_dependency'
```

[1] https://github.com/rails/rails/blob/4-2-0/activesupport/lib/active_support/dependencies.rb#L274
I think we should append `_` prefix to make it more AS-proof