Remove minimum-stability from composer.json

We do not need it since we do not have any non-dev-dependencies,
and phpunit can be the latest stable version.

Resolves: #123

Author: cweiske

composer.json

@@ -16,7 +16,6 @@
         }
     ],
     "license": "MIT",
-    "minimum-stability": "dev",
     "require": {
         "php": ">=5.5",
         "ext-curl": "*",