liquid ~> 2.6.1

`landable.gemspec` should specify a version of `liquid`.

Liquid 3.0.0 pretty much fails all the specs:

```
  31) Landable::Api::PagesController#index renders pages as json
     Failure/Error: let(:pages) { @pages ||= create_list(:page, 5) }
     ActiveRecord::RecordInvalid:
       Validation failed: Body had a problem: private method `new' called for Landable::Liquid::TitleTag:Class
     # ./spec/controllers/landable/api/pages_controller_spec.rb:56:in `block (3 levels) in <module:Api>'
     # ./spec/controllers/landable/api/pages_controller_spec.rb:57:in `block (3 levels) in <module:Api>'
```