Set event_name to visible on web and system hooks (#360)

Author: PhilippeVienne

src/main/java/org/gitlab4j/api/systemhooks/SystemHookEvent.java

@@ -5,6 +5,7 @@
 import com.fasterxml.jackson.annotation.JsonTypeInfo;
 
 @JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
+    visible = true,
     property="event_name")
 @JsonSubTypes({
     @JsonSubTypes.Type(value = CreateProjectSystemHookEvent.class, name = ProjectSystemHookEvent.PROJECT_CREATE_EVENT),

src/main/java/org/gitlab4j/api/webhook/Event.java

@@ -6,6 +6,7 @@
 
 @JsonTypeInfo(use=JsonTypeInfo.Id.NAME,
     include=JsonTypeInfo.As.PROPERTY,
+    visible = true,
     property="object_kind")
 @JsonSubTypes({
     @JsonSubTypes.Type(value = BuildEvent.class, name = BuildEvent.OBJECT_KIND),