Try to solve build for PHP versions 5.3 & 5.4.

.travis.yml

@@ -9,7 +9,11 @@ php:
 matrix:
   allow_failures:
   - php: hhvm
-before_script: composer install --dev
+before_script:
+- if [[ "$TRAVIS_PHP_VERSION" == "5.3" -o "$TRAVIS_PHP_VERSION" == "5.4" ]];
+  then composer install --dev --prefer-lowest; else
+  composer install --dev;
+  fi
 script: phpunit --configuration tools/phpunit
 after_script:
 - if [[ "$TRAVIS_PHP_VERSION" == "5.6" ]]; then php vendor/bin/coveralls -v; fi