GUACAMOLE-1906: Bump guac-manifest versions and add 1.5.5 to the extension loader.

guacamole/src/main/java/org/apache/guacamole/extension/ExtensionModule.java

@@ -72,7 +72,8 @@ public class ExtensionModule extends ServletModule {
             "1.5.1",
             "1.5.2",
             "1.5.3",
-            "1.5.4"
+            "1.5.4",
+            "1.5.5"
         ));
 
     /**