ci(travis): add mdspell (#394)

Add lint rule for spell checking markdown files

Author: juancarlostong

.travis.yml

@@ -35,15 +35,26 @@ stages:
 jobs:
   include:
     - stage: 'Lint markdown files'
-      language: ruby
-      rvm: 2.4.1
       os: linux
+      language: generic
       install: gem install awesome_bot
       script:
         - find . -type f -name '*.md' -exec awesome_bot {} \;
       notifications:
         email: false
 
+    - stage: 'Lint markdown files'
+      os: linux
+      language: generic
+      before_install: skip
+      install:
+        - npm i -g markdown-spellcheck
+      before_script:
+        - wget --quiet https://raw.githubusercontent.com/optimizely/mdspell-config/master/.spelling
+      script:
+        - mdspell -a -n -r --en-us '**/*.md'
+      after_success: skip
+
     - stage: 'Lint'
       node_js: '12'
       script: npm run lint