Fix Travis build memory exhaustion during composer install

(cherry picked from commit 8cab456)

Author: grimzy

.travis.yml

@@ -17,6 +17,7 @@ services:
   - docker
 
 before_install:
+  - "memory_limit=2G" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
   - sudo /etc/init.d/mysql stop
   - make start_db V=$MYSQL_VERSION