Revert minimum stability to stable, tasks #4359

Stability revert from alpha to stable. In 79c9052 set to alpha due to
earlier required composer/composer version which was not yet stable.

Composer upgrade was blocked due to changes w/ composer autoloading and
a PHP 5.3 regression allowing slash-prefixed class names in the past and
the unlucky coincidence that di.xml files made use of such class-names,
fixed in 0b243b8.

Now composer was updated in 0364bf9 and alpha degradation can be lifted.

Refs:

- #4359

- 79c9052

- 0b243b8

- 0364bf9

Author: ktomk

composer.json

@@ -258,6 +258,5 @@
             "Magento\\TestFramework\\Utility\\": "dev/tests/static/framework/Magento/TestFramework/Utility/"
         }
     },
-    "minimum-stability": "alpha",
     "prefer-stable": true
 }