ci(travis): include commitlint stage

* https://github.com/conventional-changelog/commitlint
  - Lint commit messages.
  - Ensure that they are in accordance with `semantic-release` settings.
  - Use Travis CI to display errors and prevent release stage if so.
* https://conventional-changelog.github.io/commitlint/#/reference-rules
  - Specific rules are defined in `commitlint.config.js`.
  - Using the default rules at the time of this commit.
  - This link gives full details on the default values and how to
    provide custom settings.

.travis.yml

@@ -1,3 +1,9 @@
+stages:
+  - test
+  - commitlint
+  - name: release
+    if: branch = master
+
 sudo: required
 cache: bundler
 language: ruby
@@ -22,9 +28,16 @@ before_deploy:
   - test $TRAVIS_TEST_RESULT = 0
 
 jobs:
-  # Only run if this is the `master` branch
-  if: branch = master
   include:
+    # Define the commitlint stage
+    - stage: commitlint
+      language: node_js
+      node_js: lts/*
+      before_install: skip
+      script:
+        - npm install @commitlint/config-conventional -D
+        - npm install @commitlint/travis-cli -D
+        - commitlint-travis
     # Define the release stage that runs semantic-release
     - stage: release
       language: node_js

commitlint.config.js

@@ -0,0 +1,3 @@
+module.exports = {
+    extends: ['@commitlint/config-conventional'],
+};