Updated settings import in contrib/feedexport.py class S3FeedStorage

This little patch fixes the use of the deprecated scrapy.settings module used in the class S3FeedStorage in contrib/feedexport.py. After applying this commit the warning below will not appear when exporting to S3. 

    /usr/lib/python2.7/site-packages/Scrapy-0.17.0-py2.7.egg/scrapy/contrib/feedexport.py:84: ScrapyDeprecationWarning: Module `scrapy.conf` is deprecated, use `crawler.settings` attribute instead