Update composer

.travis.yml

@@ -16,6 +16,7 @@ matrix:
     fast_finish: true
 
 before_script:
+  - composer self-update
   - if [ "`phpenv version-name`" != "hhvm" ]; then echo "extension = mongo.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi
   - if [ "`phpenv version-name`" != "hhvm" ]; then echo "extension = amqp.so" >> ~/.phpenv/versions/$(phpenv version-name)/etc/php.ini; fi
   - if [ "$deps" == "low" ]; then composer install --prefer-source --prefer-lowest --prefer-stable; fi