add 'equals' to the filter predicates that default to string

as discovered by @JakeAustwick in IRC:

https://botbot.me/freenode/activeadmin/msg/9660435/
refactor tests for string filters w/ a predicate (e.g. body_equals)
lock Travis icon in README to the master branch
Add ability to order by hstore columns in PostgreSQL

Example:

```ruby
class Device < ActiveRecord::Base
  store_accessor :information, :model
end
```

```ruby
ActiveAdmin.register Device do
  index do
    column :model, sortable: "information->'model'"
  end
end
```
Merge pull request #2844 from AndreyChernyh/f/add-hstore-sort

Add ability to order by hstore columns in PostgreSQL
#2414: don't accidentally set up AA routes multiple times
Support scopes with spaces in the name
clarify the state of the project in the README
Check show action for id column in index
update Kaminari version requirement for 0.15.1
Merge pull request #2870 from quainjn/2869-index-id-without-show

Check show action for id column in index
Merge pull request #2868 from phiyangt/2867-fix-scope-id-collisions

Support scopes with spaces in the name
add version and license badges to README
Update fr.yml
update CHANGELOG formatting
Only reorder collection if it is grouped

Only need to use work around of GROUP BY + COUNT queries if we are actually using grouping.

This is a squash-merge of #2864
simplify `collection_size` after #2864
#2382: add test for belongs_to filter w/ custom foreign key

Ransack handles it for us, so there's no need to do all that extra work
:D

Note that this commit disables part of `new_page.feature` as a
side-effect of me renaming the association's foreign key.
Improve Draper integration proof-of-concept [ci skip]
This "works", but we should not need to check if the collection is already decorated.

I found that the collection has already been decorated in lib/active_admin/resource_controller/data_access.rb; this still need some research...