Server: fixed broken Travis build

.travis.yml

@@ -26,6 +26,7 @@ before_install:
 install:
     - composer install --dev --prefer-dist
     - composer require react/promise:2.*
+    - composer require psr/http-message:1.*
 
 script: if [ "$TRAVIS_PHP_VERSION" == "5.6" ]; then bin/phpunit --coverage-clover build/logs/clover.xml --group default,ReactPromise; else bin/phpunit --group default,ReactPromise; fi
 

composer.json

@@ -32,6 +32,7 @@
     }
   },
   "suggest": {
-    "react/promise": "To use ReactPHP promise adapter"
+    "react/promise": "To leverage async resolving on React PHP platform",
+    "psr/http-message": "To use standard GraphQL server"
   }
 }