Adds script linting to Travis

.travis.yml

@@ -1,7 +1,14 @@
 language: php
 
-matrix:
+cache:
+	apt: true
+	yarn: true
+	directories:
+		- $HOME/.composer/cache/files
+		- $HOME/.yarn/node_modules
+	- node_modules
 
+matrix:
     include:
         # aliased to a recent 5.6.x version
         - php: '5.6'
@@ -41,6 +48,8 @@ before_script:
     - if [[ "$SNIFF" == "1" ]]; then phpenv rehash; fi
     # Set up unit tests
     - bash app/bin/install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
+	# Install npm modules
+	- yarn
 
 script:
     # Search for PHP syntax errors.
@@ -56,4 +65,6 @@ script:
     # --extensions: Only sniff PHP files.
     - if [[ "$SNIFF" == "1" ]]; then $PHPCS_DIR/scripts/phpcs -p -s -v -n . --standard="WordPress-VIP" --extensions=php; fi
     # Run unit tests
-    - phpunit
+    - phpunit
+	# Run linting
+	- npm run lint-scripts