Revert contract dependency change

Author: driesvints

composer.json

@@ -21,6 +21,7 @@
         "illuminate/auth": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
         "illuminate/console": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
         "illuminate/container": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
+        "illuminate/contracts": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
         "illuminate/database": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
         "illuminate/encryption": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
         "illuminate/http": "~5.6.0|~5.7.0|~5.8.0|~5.9.0",
@@ -31,7 +32,6 @@
         "zendframework/zend-diactoros": "~1.0"
     },
     "require-dev": {
-        "illuminate/contracts": "dev-master",
         "mockery/mockery": "~1.0",
         "phpunit/phpunit": "~7.4"
     },