Prefer dist + cache for Travis builds

To speed up builds

.travis.yml

@@ -10,6 +10,11 @@ php:
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
 sudo: false
 
+## Cache composer
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 matrix:
   include:
     - php: 5.5
@@ -17,7 +22,7 @@ matrix:
 
 before_script:
   - travis_retry composer self-update
-  - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-source
+  - travis_retry composer update ${COMPOSER_FLAGS} --no-interaction --prefer-dist
 
 script:
   - vendor/bin/phpcs --standard=psr2 src/