Updated to Circle CI 2.0.0 configuration.

Uses latest configuration version in order to not be caught off-guard
if/when they drop v1.0.0 support.

[Details](https://circleci.com/docs/2.0).

circle.yml

@@ -1,7 +1,33 @@
-dependencies:
+version: 2
-  pre:
+jobs:
-    - gem update --system
+  build:
-    - gem update bundler
+    working_directory: ~/project
-test:
+    docker:
-  override:
+      - image: circleci/ruby:2.5.0
-    - bundle exec rake
+        environment:
+          BUNDLE_JOBS: 3
+          BUNDLE_RETRY: 3
+          BUNDLE_PATH: vendor/bundle
+          EDITOR: vim
+    steps:
+      - checkout
+
+      - type: cache-restore
+        name: Bundler Cache Restore
+        key: bundle-{{ checksum "Gemfile" }}
+
+      - run:
+          name: Bundler Install
+          command: |
+            gem update --system
+            bundle check || bundle install
+
+      - type: cache-save
+        name: Bundler Cache Store
+        key: bundle-{{ checksum "Gemfile" }}
+        paths:
+          - vendor/bundle
+
+      - run:
+          name: Rake Run
+          command: bundle exec rake