Set next Gerrit version to 3.1

Current stable branch is 3.0 and master is pointing to
version 3.1.

Change-Id: I648935e944fcda32a551bca3814ba152f12ea7af

src/main/java/com/googlesource/gerrit/plugins/manager/GerritVersionBranch.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/manager/GerritVersionBranch.java b/src/main/java/com/googlesource/gerrit/plugins/manager/GerritVersionBranch.java
index 6a36133..e138e92 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/manager/GerritVersionBranch.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/manager/GerritVersionBranch.java
@@ -15,7 +15,7 @@
 package com.googlesource.gerrit.plugins.manager;
 
 public class GerritVersionBranch {
-  private static final String GERRIT_NEXT_VERSION = "3.0";
+  private static final String GERRIT_NEXT_VERSION = "3.1";
 
   public static String getBranch(String gerritVersion) {
     if (gerritVersion == null