Add additional Rubies using Workflows.

A recent issue (#1010) highlighted an deprecation under certain Ruby
versions that we should be aware of. So far, we've been using CircleCI
for running tests and previously they didn't support multiple builds.

With 2.0, Workflows allows for something like this to be setup. This
uses YAML inheritance to share a job definition across multiple build
types (defined as different Docker configurations).

This introduces Ruby 2.3 in addition to 2.4.1.

.circleci/config.yml

@@ -1,7 +1,42 @@
-version: 2
+default_job: &default_job
+  working_directory: ~/administrate
+  steps:
+    - checkout
+
+    # Restore Cached Dependencies
+    - type: cache-restore
+      name: Restore bundle cache
+      key: administrate-{{ checksum "Gemfile.lock" }}
+
+    # Bundle install dependencies
+    - run: bundle install --path vendor/bundle
+
+    # Install Appraisal gemfiles
+    - run: bundle exec appraisal install
+
+    # Cache Dependencies
+    - type: cache-save
+      name: Store bundle cache
+      key: administrate-{{ checksum "Gemfile.lock" }}
+      paths:
+        - vendor/bundle
+
+    # Wait for DB
+    - run: dockerize -wait tcp://localhost:5432 -timeout 1m
+
+    # Setup the environment
+    - run: cp .sample.env .env
+
+    # Setup the database
+    - run: bundle exec rake db:setup
+
+    # Run the tests
+    - run: bundle exec rake
+    - run: bundle exec appraisal rake
+
 jobs:
-  build:
-    working_directory: ~/administrate
+  ruby-2.4:
+    <<: *default_job
     docker:
       - image: circleci/ruby:2.4.1-node-browsers
         environment:
@@ -13,36 +48,24 @@ jobs:
           POSTGRES_USER: administrate
           POSTGRES_DB: administrate-prototype_test
           POSTGRES_PASSWORD: ""
-    steps:
-      - checkout
-
-      # Restore Cached Dependencies
-      - type: cache-restore
-        name: Restore bundle cache
-        key: administrate-{{ checksum "Gemfile.lock" }}
-
-      # Bundle install dependencies
-      - run: bundle install --path vendor/bundle
-
-      # Install Appraisal gemfiles
-      - run: bundle exec appraisal install
-
-      # Cache Dependencies
-      - type: cache-save
-        name: Store bundle cache
-        key: administrate-{{ checksum "Gemfile.lock" }}
-        paths:
-          - vendor/bundle
 
-      # Wait for DB
-      - run: dockerize -wait tcp://localhost:5432 -timeout 1m
-
-      # Setup the environment
-      - run: cp .sample.env .env
-
-      # Setup the database
-      - run: bundle exec rake db:setup
+  ruby-2.3:
+    <<: *default_job
+    docker:
+      - image: circleci/ruby:2.3-node-browsers
+        environment:
+          PGHOST: localhost
+          PGUSER: administrate
+          RAILS_ENV: test
+      - image: postgres:9.5-alpine
+        environment:
+          POSTGRES_USER: administrate
+          POSTGRES_DB: administrate-prototype_test
+          POSTGRES_PASSWORD: ""
 
-      # Run the tests
-      - run: bundle exec rake
-      - run: bundle exec appraisal rake
+workflows:
+  version: 2
+  multiple-rubies:
+    jobs:
+      - ruby-2.4
+      - ruby-2.3