fix 'type' object cannot be interpreted as an integer

PDB stacktrace:
```
 plugins/twitter_bootstrap_rst_directives/bootstrap_rst_directives.py(369)run()
-> child.set_class('alert-link')
> tmp/lib/python3.7/site-packages/docutils/nodes.py(1072)set_class()
-> DeprecationWarning, stacklevel=2)
```

The deprecated function code:
```
  1067     def set_class(self, name):
  1068         """Add a new class to the "classes" attribute."""
  1069         warnings.warn('docutils.nodes.Element.set_class() is deprecated; '
  1070                       ' and will be removed in Docutils 0.21 or later.',
  1071                       "Append to Element['classes'] list attribute directly",
  1072                       DeprecationWarning, stacklevel=2)
  1073         assert ' ' not in name
  1074         self['classes'].append(name.lower())
```

twitter_bootstrap_rst_directives/bootstrap_rst_directives.py

@@ -366,7 +366,7 @@ def run(self):
         # Recurse inside its children and change the hyperlinks classes
         for child in new_element.traverse(include_self=False):
             if isinstance(child, nodes.reference):
-                child.set_class('alert-link')
+                child['classes'].append('alert-link')
         # Set its custom bootstrap classes
         new_element['classes'] += ['alert ', alert_name]
         # Return one single element