Don't build on *.md change [skip ci]

.travis.yml

@@ -1,6 +1,14 @@
 sudo: false
 language: node_js
-
+before_install:
+- |
+    if [ "$TRAVIS_PULL_REQUEST" != "false" ]; then
+      TRAVIS_COMMIT_RANGE="FETCH_HEAD...$TRAVIS_BRANCH"
+    fi
+    git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md$)|(^(ast))/' || {
+      echo "Only docs were updated, stopping build process."
+      exit
+    }
 node_js:
   - "6"
   - "5"