FIX: Allow pre-release versions of SS4.

Since SS4 has not yet been released, and this module is used to test
pre-release versions of SS4, we need to have @dev on the requirement
to make it as flexible as possible.

As a general rule, modules that plug *into* SS4 rather than making *use*
of it should have @dev on the end of their dependencies.

composer.json

@@ -16,7 +16,7 @@
     }
   ],
   "require": {
-    "silverstripe/framework": "^4.0"
+    "silverstripe/framework": "^4.0@dev"
   },
   "extra": {
     "branch-alias": {