OF-1339: Prepare websocket plugin for merge into core.

Openfire 4.2.0 will have the functionality that's now in the websocket plugin. As a result,
this plugin should not be used in Openfire 4.2.0. To facilitate this, we've defined a new concept
of 'priorToServerVersion' in OF-1353.

This commit defines that the plugin should only be used in Openfire instances prior to version 4.2.0.

src/plugins/websocket/changelog.html

@@ -48,7 +48,9 @@ Openfire WebSocket Plugin Changelog
 
 <ul>
     <li>[<a href='https://igniterealtime.org/issues/browse/OF-1362'></a>] - OF-1362: Websocket plugin has been broken since Openfire 4.1.4.</li>
+    <li>[<a href='https://igniterealtime.org/issues/browse/OF-1353'></a>] - OF-1353: Define a 'priorToServerVersion' in preparation of <a href="https://issues.igniterealtime.org/browse/OF-1339">OF-1339: </a> Merge websocket plugin with Openfire core.</li>
     <li>Minimum server requirement: 4.1.5</li>
+    <li>Prior-to server requirement: 4.2.0</li>
 </ul>
 
 <p><b>1.2.0</b> -- June 9, 2017</p>

src/plugins/websocket/plugin.xml

@@ -12,4 +12,5 @@
     <date>08/07/2017</date>
     <url>https://tools.ietf.org/html/rfc7395</url>
     <minServerVersion>4.1.5</minServerVersion>
+    <priorToServerVersion>4.2.0</priorToServerVersion>
 </plugin>

src/plugins/websocket/readme.html

@@ -59,6 +59,10 @@ as the BOSH component, and will reuse the same HTTP/S port(s) when establishing
 
 <h2>Installation</h2>
 
+<p>Please note that the functionality provided by this plugin is <a href="https://issues.igniterealtime.org/browse/OF-1339">
+added to Openfire direclty in version 4.2.0</a>. <em>This plugin is no longer needed in Openfire 4.2.0 or later!</em>
+</p>
+
 <p>Copy websocket.jar into the plugins directory of your Openfire installation. The
 plugin will then be automatically deployed. To upgrade to a new version, copy the new
 websocket.jar file over the existing file.</p>