Update master to framework 5 compatibility

composer.json

@@ -28,8 +28,8 @@
         "behat/mink-extension": "^2.1",
         "behat/mink-selenium2-driver": "^1.3",
         "symfony/dom-crawler": "^3",
-        "silverstripe/testsession": "^2.0.0@alpha",
-        "silverstripe/framework": "^4@dev",
+        "silverstripe/testsession": "^3",
+        "silverstripe/framework": "^5",
         "symfony/finder": "^3.2"
     },
     "autoload": {
@@ -44,7 +44,7 @@
     },
     "extra": {
         "branch-alias": {
-            "3.x-dev": "3.1.x-dev"
+            "dev-master": "4.x-dev"
         }
     },
     "scripts": {