Replace ember-addon keyword with ember-codemod (#132)

* package.json: Remove `ember-addon` keyword

This is *not* an Ember addon, it is a codemod aimed at Ember projects. Using this keyword makes Ember CLI think that this is an actual addon, which can have unintended side-effects.

* package.json: Add `ember-codemod` keyword

This replaces the `ember-addon` keyword and will hopefully soon be picked up by EmberObserver

package.json

@@ -6,7 +6,7 @@
     "codemod-cli",
     "codemods",
     "ember",
-    "ember-addon"
+    "ember-codemod"
   ],
   "homepage": "https://github.com/ember-codemods/ember-angle-brackets-codemod#readme",
   "bugs": {