Add Ruby 2.2.9 to Circle and clarify support.

The version support is informed by:

* https://stackoverflow.com/a/23532476/83386
* http://guides.rubyonrails.org/5_0_release_notes.html

Closes #1010.

.circleci/config.yml

@@ -36,6 +36,20 @@ default_job: &default_job
     - run: bundle exec appraisal rake
 
 jobs:
+  ruby-2.2:
+    <<: *default_job
+    docker:
+      - image: circleci/ruby:2.2.9-node-browsers
+        environment:
+          PGHOST: localhost
+          PGUSER: administrate
+          RAILS_ENV: test
+      - image: postgres:9.5-alpine
+        environment:
+          POSTGRES_USER: administrate
+          POSTGRES_DB: ruby22
+          POSTGRES_PASSWORD: ""
+
   ruby-2.3:
     <<: *default_job
     docker:
@@ -85,3 +99,4 @@ workflows:
       - ruby-2.5
       - ruby-2.4
       - ruby-2.3
+      - ruby-2.2

README.md

@@ -42,7 +42,8 @@ To accomplish these goals, Administrate follows a few guiding principles:
 
 ## Getting Started
 
-Administrate supports Rails from 4.2, up to 5.0 and beyond.
+Administrate supports Rails from 4.2, up to 5.0 and beyond. We support Ruby
+2.2.9 and up.
 
 Add Administrate to your Gemfile and re-bundle: