Correct the invocation of ‘_assemble_from_fragments’

The previous commit (82c9f5) didn't work for me.

```
  File "/usr/local/Cellar/ansible/HEAD/lib/python2.7/site-packages/ansible/runner/__init__.py", line 394, in _executor
    exec_rc = self._executor_internal(host, new_stdin)
  File "/usr/local/Cellar/ansible/HEAD/lib/python2.7/site-packages/ansible/runner/__init__.py", line 485, in _executor_internal
    return self._executor_internal_inner(host, self.module_name, self.module_args, inject, port, complex_args=complex_args)
  File "/usr/local/Cellar/ansible/HEAD/lib/python2.7/site-packages/ansible/runner/__init__.py", line 685, in _executor_internal_inner
    result = handler.run(conn, tmp, module_name, module_args, inject, complex_args)
  File "/usr/local/Cellar/ansible/HEAD/lib/python2.7/site-packages/ansible/runner/action_plugins/assemble.py", line 67, in run
    path = _assemble_from_fragments(src, delimiter)
NameError: global name '_assemble_from_fragments' is not defined
```

This PR got rid of that error though. I'm not sure if this is the correct way to fix it though.