Use --prefer-dist instead of --prefer-source, as suggested in https://twitter.com/seldaek/status/708236348281495552

.travis.yml

@@ -13,7 +13,7 @@ matrix:
       env: WP_VERSION=latest WP_MULTISITE=1
 before_script:
   - ./install-wp-tests.sh wordpress_test root '' localhost $WP_VERSION
-  - composer install --working-dir=wp-add-to-homescreen --prefer-source --no-interaction
+  - composer install --working-dir=wp-add-to-homescreen --prefer-dist --no-interaction
   - npm install -g bower
   - cd wp-add-to-homescreen && bower install && cd ..
 

README.md

@@ -12,7 +12,7 @@ First clone the repository.
 Once inside the repository you will need [composer](https://getcomposer.org) and [bower](http://bower.io/) installed. Go to the folder `wp-add-to-homescreen` inside the repository folder and run:
 
 ```
-$ composer install --prefer-source --no-interaction
+$ composer install
 $ bower install
 ```