Activeadmin v3 support

[difernandez]

Motivation / Background

This Pull Request has been created because ActiveAdmin released v3 🎉 . It has some breaking changes, including requiring of ransack v4, so some changes needed to be done on our part to support it.

Detail

This Pull Request applies a couple of backwards compatible changes that allow this gem to work well with AA v3, and for tests to pass using that version:

• Adds ransackable_attributes and ransackable_associations to dummy app models, as it is now explicitly required by ransack v4
• Removes some filter predicates (pretty much just contains), that are now removed from AA. These predicates were backports in AA, and they have been replaced by their already existent Ransack equivalents
• Updates activeadmin version in Gemfile, to test and develop using v3

Additional information

Closes #475

Checklist

Before submitting the PR make sure the following are checked:

• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [x] Commit message has a concise description of what changed and why.
• [x] Tests are added or updated if you fix a bug or add a feature.
• [x] Documentation has been added or updated if you add a feature or modify an existing one.
• [x] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature (under the "Unreleased" heading if this is not a version change).
• [x] My changes don't introduce any linter rule violations.

[jgmontoya]

Can we get a release of this before the whole v2 overhaul?

[difernandez]

@jgmontoya even though we won't be backporting things to v1, this seems important enough that we can release it for a last v1 update. I opened #479 for this, so we should be releasing that in the short run

[difernandez]

@jgmontoya we just released v1.10.1 , please let us know how it goes if you try it out