Internal: cancel build with only .md changes (#4547)

.travis.yml

@@ -5,6 +5,15 @@ language: node_js
 cache:
   directories:
   - node_modules
+before_install:
+- |
+    if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+      TRAVIS_COMMIT_RANGE="FETCH_HEAD...$TRAVIS_BRANCH"
+    fi
+    git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md$)|(^(doc))/' || {
+      echo "Only docs were updated, stopping build process."
+      exit
+    }
 node_js:
 - '6'
 - '5'