Resolve plugin dependency chains with XEP-0115.

The post_init() system can only reliably handle a single layer
of dependencies between plugins, but PEP plugins with XEP-0115
exceed that limit and plugins can be post_init'ed out of order. To
resolve this, we will special case XEP-0115 to be post_init'ed
first until the new plugin system with dependency tracking is
stable.

sleekxmpp/basexmpp.py

@@ -185,6 +185,16 @@ class BaseXMPP(XMLStream):
         - The send queue processor
         - The scheduler
         """
+
+        # The current post_init() process can only resolve a single
+        # layer of inter-plugin dependencies. However, XEP-0115 and
+        # plugins which depend on it exceeds this limit and can cause
+        # failures if plugins are post_inited out of order, so we must
+        # manually process XEP-0115 first.
+        if 'xep_0115' in self.plugin:
+            if not self.plugin['xep_0115'].post_inited:
+                self.plugin['xep_0115'].post_init()
+
         for name in self.plugin:
             if not self.plugin[name].post_inited:
                 self.plugin[name].post_init()
@@ -257,10 +267,6 @@ class BaseXMPP(XMLStream):
             else:
                 raise NameError("Plugin %s not in plugins.__all__." % plugin)
 
-        # Resolve plugin inter-dependencies.
-        for plugin in self.plugin:
-            self.plugin[plugin].post_init()
-
     def __getitem__(self, key):
         """Return a plugin given its name, if it has been registered."""
         if key in self.plugin: