Release 6.1.0 (#391)

- Adds `subdomain` for `merchants`
- Adds new field `challenge_notification_url` to the `completion object`
- Fix `payment context details` response

gradle.properties

@@ -1,5 +1,5 @@
 group=com.checkout
-version=6.0.32
+version=6.1.0
 
 maven2_url=https://oss.sonatype.org/service/local/staging/deploy/maven2/
 snapshot_url=https://oss.sonatype.org/content/repositories/snapshots/