Drop support for Ember.js below v3.4.0

This is not strictly necessary since the addon probably still works fine, but to keep our CI times reasonable we will officially drop support for versions that are not even supported by the Ember team itself anymore.

package.json

@@ -54,7 +54,7 @@
   "ember-addon": {
     "configPath": "tests/dummy/config",
     "versionCompatibility": {
-      "ember": ">=2.0.0"
+      "ember": ">=3.4.0"
     }
   }
 }