feat(loadSpec): change the endpoint called by loadSpec (#356)

According to this Kubernetes doc:
https://kubernetes.io/docs/concepts/overview/kubernetes-api/#openapi-and-swagger-definitions

The `/swagger*` endpoints have been deprecated starting with v1.10.

This commit changes `loadSpec` to address this:
1. Initially attempts to contact `/openapi/v2` with `Accept: application/json` in order
   to get the swagger file via the new URL
2. If that fails with a 404, attempts to contact `swagger.json` to support clusters which
   do not have the new endpoint

lib/swagger-client.js

@@ -39,13 +39,22 @@ class Root extends Component {
    */
   loadSpec() {
     return new Promise((resolve, reject) => {
-      this.http.request('GET', { path: '/swagger.json' }, (err, res) => {
-        if (err) return reject(err);
-        if (res.statusCode !== 200) {
-          return reject(new Error(`Failed to get swagger.json: ${res.statusCode}`));
+      return this.http.request('GET', { path: '/openapi/v2' }, (modernGETErr, modernGETRes) => {
+        if (modernGETErr) return reject(modernGETErr);
+        if (modernGETRes.statusCode === 404) {
+          return this.http.request('GET', { path: '/swagger.json' }, (legacyGETErr, legacyGETRes) => {
+            if (legacyGETErr) return reject(legacyGETErr);
+            if (legacyGETRes.statusCode !== 200) {
+              return reject(new Error(`Failed to get /swagger.json: ${legacyGETRes.statusCode}`));
+            }
+            this._addSpec(legacyGETRes.body);
+            return resolve(this);
+          });
+        } else if (modernGETRes.statusCode !== 200) {
+          return reject(new Error(`Failed to get /openapi/v2: ${modernGETRes.statusCode}`));
         }
-        this._addSpec(res.body);
-        resolve(this);
+        this._addSpec(modernGETRes.body);
+        return resolve(this);
       });
     });
   }