travis: testing with lowest dependencies

.travis.yml

@@ -11,6 +11,10 @@ matrix:
  - php: 7.0
  - php: hhvm
 
+ include:
+ - php: 5.6
+ env: dependencies="--prefer-lowest --prefer-stable"
+
 script:
  - vendor/bin/tester tests -s -p php -c tests/php-unix.ini
  - php temp/code-checker/src/code-checker.php --short-arrays
@@ -21,7 +25,7 @@ after_failure:
 
 before_script:
  # Install Nette Tester & Code Checker
- - composer install --no-interaction --prefer-source
+ - composer update --no-interaction --prefer-source $dependencies
  - composer create-project nette/code-checker temp/code-checker ~2.5 --no-interaction --prefer-source
 
  # Create databases.ini