Add caching on travis builds

.travis.yml

@@ -2,6 +2,10 @@ language: php
 
 sudo: false
 
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 matrix:
     include:
         - php: 7.0
@@ -15,7 +19,7 @@ before_script:
   - echo "extension = redis.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
   - echo "extension = mongodb.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
   - echo "extension = amqp.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
-  - if [ "$deps" == "low" ]; then composer update --prefer-source --prefer-lowest --prefer-stable; fi
-  - if [ "$deps" != "low" ]; then composer install --prefer-source; fi
+  - if [ "$deps" == "low" ]; then composer update --prefer-dist --prefer-lowest --prefer-stable; fi
+  - if [ "$deps" != "low" ]; then composer install --prefer-dist; fi
 
 script: composer test