Update .travis.yml

Final change to run sample codes in PHP 7

Author: ashtru

.travis.yml

@@ -14,7 +14,7 @@ before_script:
 script:
     - vendor/phpunit/phpunit/phpunit --coverage-clover=coverage.clover
     - cd sample-code-php
-    - composer update
+    - composer install --prefer-dist --ignore-platform-reqs
     - vendor/phpunit/phpunit/phpunit test-runner.php .
 
 after_script: