remove stability settings from composer.json

With `prefer-stable` set to `true`, the `DEPENDECIES=dev` build job
will (almost) never install not yet released dependencies. Without
the `prefer-stable` setting we should also drop the `minimum-stability`
setting (which is now useless as the config value is set in the Travis
job).

Author: xabbuh

composer.json

@@ -55,8 +55,6 @@
     "conflict": {
         "php-http/guzzle6-adapter": "<1.1"
     },
-    "minimum-stability": "dev",
-    "prefer-stable": true,
     "autoload": {
         "psr-4": {
             "Http\\HttplugBundle\\": ""