Updated to Circle CI 2.1.0

Necessary to pick up the new syntax, configuration location, and custom
Docker image.

.circleci/config.yml

@@ -0,0 +1,31 @@
+version: 2.1
+jobs:
+  build:
+    working_directory: ~/project
+    docker:
+      - image: bkuhlmann/ruby:latest
+    steps:
+      - checkout
+
+      - restore_cache:
+          name: Bundler Restore
+          keys:
+            - gem-cache-{{.Branch}}-{{checksum "Gemfile"}}
+            - gem-cache-
+
+      - run:
+          name: Bundler Install
+          command: |
+            gem update --system
+            bundle config set path "vendor/bundle"
+            bundle install
+
+      - save_cache:
+          name: Bundler Store
+          key: gem-cache-{{.Branch}}-{{checksum "Gemfile"}}
+          paths:
+            - vendor/bundle
+
+      - run:
+          name: Build
+          command: bundle exec rake

circle.yml

@@ -1,62 +0,0 @@
-version: 2
-jobs:
-  build:
-    working_directory: ~/project
-    docker:
-      - image: circleci/ruby
-        environment:
-          BUNDLE_JOBS: 3
-          BUNDLE_RETRY: 3
-          BUNDLE_PATH: vendor/bundle
-          EDITOR: vim
-    steps:
-      - checkout
-
-      - run:
-          name: Environment Setup
-          command: |
-            printf "%s\n" 'export CI_RUBY_VERSION=$(cat ".ruby-version" | tr -d "\n")' >> $BASH_ENV
-
-      - type: cache-restore
-        name: Ruby Restore
-        key: ruby-{{checksum ".ruby-version"}}
-
-      - run:
-          name: Ruby Install
-          command: |
-            cd ..
-            curl https://cache.ruby-lang.org/pub/ruby/${CI_RUBY_VERSION::-2}/ruby-$CI_RUBY_VERSION.tar.gz > ruby-$CI_RUBY_VERSION.tar.gz
-            tar --extract --gzip --verbose --file ruby-$CI_RUBY_VERSION.tar.gz
-            cd ruby-$CI_RUBY_VERSION
-            ./configure
-            make
-            make update-gems
-            make extract-gems
-            sudo make install
-
-      - type: cache-save
-        name: Ruby Store
-        key: ruby-{{checksum ".ruby-version"}}
-        paths:
-          - ../ruby-$CI_RUBY_VERSION
-
-      - type: cache-restore
-        name: Bundler Restore
-        key: bundler-{{checksum "Gemfile"}}
-
-      - run:
-          name: Bundler Install
-          command: |
-            gem update --system
-            bundle config set path "vendor/bundle"
-            bundle install
-
-      - type: cache-save
-        name: Bundler Store
-        key: bundler-{{checksum "Gemfile"}}
-        paths:
-          - vendor/bundle
-
-      - run:
-          name: Build
-          command: bundle exec rake