Fix Nokogiri deprecation warning

Nokogiri warning:

```
warning: Passing a Node as the second parameter to Node.new is deprecated. Please pass a Document instead, or prefer an alternative constructor like Node#add_child. This will become an error in a future release of Nokogiri.
```

Please see the following for more details:
sparklemotion/nokogiri#975

lib/gollum-lib/filter/toc.rb

@@ -125,7 +125,7 @@ def generate_anchor_name(header)
   # Creates an anchor element with the given name and adds it before
   # the given header element.
   def add_anchor_to_header(header, name)
-    a = Nokogiri::XML::Node.new('a', @doc)
+    a = Nokogiri::XML::Node.new('a', @doc.document)
     a['class'] = 'anchor'
     a['id'] = name
     a['href'] = "##{name}"
@@ -143,17 +143,17 @@ def add_entry_to_toc(header, name)
 
     if @tail_level < level
       while @tail_level < level
-        list = Nokogiri::XML::Node.new('ul', @doc)
+        list = Nokogiri::XML::Node.new('ul', @doc.document)
         @tail.add_child(list)
-        @tail = list.add_child(Nokogiri::XML::Node.new('li', @doc))
+        @tail = list.add_child(Nokogiri::XML::Node.new('li', @doc.document))
         @tail_level += 1
       end
     else
       while @tail_level > level
         @tail = @tail.parent.parent
         @tail_level -= 1
       end
-      @tail = @tail.parent.add_child(Nokogiri::XML::Node.new('li', @doc))
+      @tail = @tail.parent.add_child(Nokogiri::XML::Node.new('li', @doc.document))
     end
 
     # % -> %25 so anchors work on Firefox. See issue #475