don't check meaningless "ok" field in output from Tire config endpoint

its value is always true regardless of cluster health, and it is removed
in ES v1.x.

app.rb

@@ -192,7 +192,7 @@ get '/heartbeat' do
   es_ok = false
   begin
     es_status = get_es_status
-    es_ok = ( es_status["ok"] == true and es_status["status"] == 200 )
+    es_ok = es_status["status"] == 200
   rescue
   end
   error 500, JSON.generate({"OK" => false, "check" => "es"}) unless es_ok

spec/app_spec.rb

@@ -106,15 +106,11 @@ describe "app" do
       end
 
       it "reports success when es is ready" do
-        test_es_check({"ok" => true, "status" => 200}, true)
-      end
-
-      it "reports failure when es is not ok" do
-        test_es_check({"ok" => false, "status" => 200}, false)
+        test_es_check({"status" => 200}, true)
       end
 
       it "reports failure when es status is unexpected" do
-        test_es_check({"ok" => true, "status" => 201}, false)
+        test_es_check({"status" => 503}, false)
       end
 
       it "reports failure when es status is malformed" do