refactor(iam): Use proper default IAM URL

Author: lpatino10

README.md

@@ -180,7 +180,7 @@ When authenticating with IAM, you have the option of passing in:
 // in the constructor, letting the SDK manage the IAM token
 IamOptions options = new IamOptions.Builder()
   .apiKey("<iam_api_key>")
-  .url("<iam_url>") // optional - the default value is https://iam.ng.bluemix.net/identity/token
+  .url("<iam_url>") // optional - the default value is https://iam.bluemix.net/identity/token
   .build();
 Discovery service = new Discovery("2017-11-07", options);
 ```

core/src/main/java/com/ibm/watson/developer_cloud/service/security/IamTokenManager.java

@@ -34,7 +34,7 @@ public class IamTokenManager {
   private IamToken tokenData;
 
   private static final String DEFAULT_AUTHORIZATION = "Basic Yng6Yng=";
-  private static final String DEFAULT_IAM_URL = "https://iam.ng.bluemix.net/identity/token";
+  private static final String DEFAULT_IAM_URL = "https://iam.bluemix.net/identity/token";
   private static final String GRANT_TYPE = "grant_type";
   private static final String REQUEST_GRANT_TYPE = "urn:ibm:params:oauth:grant-type:apikey";
   private static final String REFRESH_GRANT_TYPE = "refresh_token";