Fix ImportError while importing django's `render_to_string`

[claymation]

Fixes:

Traceback (most recent call last):
  File "../../lib/python2.7/site-packages/djangosphinx/config.py", line 56, in <module>
    print render_to_string(getattr(settings, 'SPHINX_CONFIG_TEMPLATE', 'conf/sphinx.conf'), context)
  File "../../lib/python2.7/site-packages/djangosphinx/config.py", line 29, in render_to_string
    return shortcuts.render_to_string(template, context, context_instance)
AttributeError: 'module' object has no attribute 'render_to_string'

Also, I'm removing the render_to_string function defined here, since we don't have a request object during config generation and using RequestContext is not necessary.