Add Ubuntu libpng16-dev for image-webpack-loader > v4

Author: flexdinesh

.travis.yml

@@ -4,6 +4,10 @@ node_js:
   - 8
   - 6
 
+before_install:
+  - sudo apt-get -qq update
+  - sudo apt-get install libpng16-dev
+  
 script:
   - npm run test
   - npm run build