Travis: improve caching between builds

Unless there is a specific reason to use `--prefer-source`, always use `--prefer-dist` (default) for the `composer install` command to allow Travis to optimally use the caching feature.

Author: jrfnl

.travis.yml

@@ -24,7 +24,7 @@ cache:
     - $HOME/.composer/cache
 
 before_script:
-  - composer install --no-interaction --prefer-source
+  - composer install --no-interaction
 
 script:
   - ant