Add dev requirement for symfony/yaml

php coveralls is failing because of this package is missing. I'm adding the dev requirement to see if it fixes the issue

Author: chadicus

composer.json

@@ -41,7 +41,8 @@
     "require-dev": {
         "php-coveralls/php-coveralls": "^2.0",
         "phpunit/phpunit": "^6.0",
-        "squizlabs/php_codesniffer": "^3.2"
+        "squizlabs/php_codesniffer": "^3.2",
+        "symfony/yaml": "^3.4"
     },
     "autoload": {
         "psr-4": { "TraderInteractive\\": "src/" }