Set next version to 3.1

Next forthcoming Gerrit version is 3.1 and thus the plugin-manager
should return master plugins when the version string starts with
3.1 *ONLY*.

Bug: Issue 11880
Change-Id: I49e193c75d2937b2ddb8dd67c1d84bf8b4e52bb9

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 debe679..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 = "2.16";
+  private static final String GERRIT_NEXT_VERSION = "3.1";
 
   public static String getBranch(String gerritVersion) {
     if (gerritVersion == null