Bump next Gerrit version to 2.16

Update the expected next version of Gerrit so that
current v2.15.x would get the plugins from a stable-2.15
branch and not from master.

Bug: Issue 8687
Change-Id: I4e269cb4b21e2e3e2980f9d6b2c8e7dea7d1fab5

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 ccf3e69..debe679 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 = "2.15";
+  private static final String GERRIT_NEXT_VERSION = "2.16";
 
   public static String getBranch(String gerritVersion) {
     if (gerritVersion == null