Prevent Travis running out of memory during composer install

.travis.yml

@@ -5,12 +5,13 @@ sudo: false
 cache:
     directories:
         - $HOME/.composer/cache
+        - vendor
 
 matrix:
     include:
         - php: 5.4
           env:
-              - DEPENDENCIES='low'
+              - COMPOSER_FLAGS='--prefer-lowest'
         - php: 5.5
         - php: 5.6
           env:
@@ -26,9 +27,10 @@ matrix:
 before_install:
     - phpenv config-rm xdebug.ini
     - composer self-update
-    - if [[ "$DEPENDENCIES" != "low" && ! $SYMFONY_VERSION ]]; then composer update -n --prefer-source; fi;
-    - if [[ "$DEPENDENCIES" != "low" && $SYMFONY_VERSION ]]; then composer require symfony/symfony:${SYMFONY_VERSION} -n --prefer-source; fi;
-    - if [[ "$DEPENDENCIES" == "low" ]]; then composer update --prefer-lowest -n --prefer-source; fi;
+    - if [[ $SYMFONY_VERSION ]]; then composer require symfony/symfony:${SYMFONY_VERSION} -n --no-update; fi;
+    # Avoid issues on composer update
+    - echo "memory_limit=-1" >> ~/.phpenv/versions/$(phpenv version-name)/etc/conf.d/travis.ini
+    - composer update $COMPOSER_FLAGS
 
 install:
     - wget http://selenium-release.storage.googleapis.com/2.46/selenium-server-standalone-2.46.0.jar