fix deprecation computed-property.override by adding setters

see https://emberjs.com/deprecations/v3.x#toc_computed-property-override

addon/components/docs-viewer/x-nav/component.js

@@ -27,7 +27,13 @@ export default Component.extend({
   /*
     This is overwritten for the Sandbox.
   */
-  project: computed(function() {
-    return this.get('store').peekRecord('project', projectName);
+  project: computed({
+    get() {
+      return this.get('store').peekRecord('project', projectName);
+    },
+
+    set(key, val) {
+      return val;
+    }
   })
 });

addon/services/project-version.js

@@ -42,22 +42,27 @@ export default Service.extend({
     return rootURL.replace(`/${this.get('currentVersion.path')}/`, '/');
   }),
 
-  currentVersion: computed(function() {
-    let config = getOwner(this).resolveRegistration('config:environment')['ember-cli-addon-docs'];
-    let currentVersion = config.deployVersion;
-
-    // In development, this token won't have been replaced replaced
-    if (currentVersion === 'ADDON_DOCS_DEPLOY_VERSION') {
-      currentVersion = {
-        key: latestVersionName,
-        name: latestVersionName,
-        tag: config.projectTag,
-        path: '',
-        sha: 'abcde'
-      };
-    }
+  currentVersion: computed({
+    get() {
+      let config = getOwner(this).resolveRegistration('config:environment')['ember-cli-addon-docs'];
+      let currentVersion = config.deployVersion;
 
-    return currentVersion;
-  })
+      // In development, this token won't have been replaced replaced
+      if (currentVersion === 'ADDON_DOCS_DEPLOY_VERSION') {
+        currentVersion = {
+          key: latestVersionName,
+          name: latestVersionName,
+          tag: config.projectTag,
+          path: '',
+          sha: 'abcde'
+        };
+      }
+
+      return currentVersion;
+    },
 
+    set(key, val) {
+      return val;
+    }
+  })
 });