[maven-release-plugin] prepare release github-api-1.13

expose issue_updated_at. It looks like a better representation of update

pom.xml

@@ -7,7 +7,7 @@
   </parent>
 
   <artifactId>github-api</artifactId>
-  <version>1.12</version>
+  <version>1.13</version>
   <name>GitHub API for Java</name>
   <url>http://github-api.kohsuke.org/</url>
   <description>GitHub API for Java</description>

src/main/java/org/kohsuke/github/GHPullRequest.java

@@ -70,6 +70,10 @@ public class GHPullRequest extends GHIssue {
         return head;
     }
 
+    public Date getIssueUpdatedAt() {
+        return GitHub.parseDate(issue_updated_at);
+    }
+
     /**
      * The HTML page of this pull request,
      * like https://github.com/jenkinsci/jenkins/pull/100