CI against Ruby 2.7

Ruby 2.7.0 has been released and this Ruby version is available on
`circleci/ruby:2.7` image.

- https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/
- https://hub.docker.com/r/circleci/ruby/tags/

```console
% docker pull circleci/ruby:2.7
2.7: Pulling from circleci/ruby

(snip)

Status: Downloaded newer image for circleci/ruby:2.7
```

.circleci/config.yml

@@ -136,6 +136,29 @@ jobs:
     steps:
       *rubocop_steps
 
+  # Ruby 2.7
+  ruby-2.7-spec:
+    docker:
+      - image: circleci/ruby:2.7
+    environment:
+      <<: *common_env
+    steps:
+      *spec_steps
+  ruby-2.7-ascii_spec:
+    docker:
+      - image: circleci/ruby:2.7
+    environment:
+      <<: *common_env
+    steps:
+      *ascii_spec_steps
+  ruby-2.7-rubocop:
+    docker:
+      - image: circleci/ruby:2.7
+    environment:
+      <<: *common_env
+    steps:
+      *rubocop_steps
+
   # ruby-head (nightly snapshot build)
   ruby-head-spec:
     docker:
@@ -217,7 +240,7 @@ jobs:
       - run:
           name: Upload coverage results to Code Climate
           command: |
-            ./tmp/cc-test-reporter sum-coverage tmp/codeclimate.*.json --parts 5 --output tmp/codeclimate.total.json
+            ./tmp/cc-test-reporter sum-coverage tmp/codeclimate.*.json --parts 6 --output tmp/codeclimate.total.json
             ./tmp/cc-test-reporter upload-coverage --input tmp/codeclimate.total.json
 
   # Miscellaneous tasks
@@ -262,6 +285,11 @@ workflows:
             - cc-setup
       - ruby-2.6-ascii_spec
       - ruby-2.6-rubocop
+      - ruby-2.7-spec:
+          requires:
+            - cc-setup
+      - ruby-2.7-ascii_spec
+      - ruby-2.7-rubocop
       - ruby-head-spec:
           requires:
             - cc-setup
@@ -279,4 +307,5 @@ workflows:
             - ruby-2.4-spec
             - ruby-2.5-spec
             - ruby-2.6-spec
+            - ruby-2.7-spec
             - jruby-9.2-spec