Adds mongodb service to Travis CI config

Author: prolificchrisb

.travis.yml

@@ -11,5 +11,9 @@ php:
   - '5.5'
   - '5.6'
 
+services: mongodb
+
+before_script:
+  - sleep 15
 before_install: echo "extension = mongo.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini
 install: travis_retry composer update --no-interaction --no-progress --no-suggest --prefer-stable