Added missing `require` to ActionView::LookupContext

Reason:

```
require 'action_view'

ActionView::LookupContext.new('views')

=> undefined method `mattr_accessor' for ActionView::LookupContext:Class (NoMethodError)
```

This pull request added missing `require 'active_support/core_ext/module/attribute_accessors'` to `ActionView::LookupContext`.