Add stable version of the testbench package

Author: vinkla

composer.json

@@ -15,7 +15,7 @@
     "require-dev": {
         "phpunit/phpunit": "~4.0",
         "mockery/mockery": "~0.9",
-        "orchestra/testbench": "~3.0"
+        "orchestra/testbench": "3.1.*"
     },
     "autoload": {
         "psr-4": {