rss_converter_twitter.com.xsl: update XPath of tweet content

rss_converter_twitter.com.xsl: update XPath of tweet content

Using the role attribute to differentiate between original tweets and
quoted tweet, as introduced in commit 4c2e986, does not work anymore,
but the fact that original tweets are <li></li> elements while quoted
tweets are <div></div> elements can be used instead.