index.html: Link to sql-novice-survey data

These are HTTPS versions of the URLs suggested in the lesson docs
since swcarpentry/sql-novice-survey@ff0f411f (Making lesson conform to
template, 2015-02-12).  I prefer HTTPS links for security, and while
files.software-carpentry.org has had an invalid certificate in the
past [1], it works now (2016-03-23).  As a fallback, you could use
[2,3], but they seem too different from the lesson docs for me to
prefer them here.

[1]: swcarpentry/DEPRECATED-site#509
[2]: https://github.com/swcarpentry/files/raw/master/survey.db
[3]: https://github.com/swcarpentry/files/raw/master/survey.sql

index.html

@@ -728,4 +728,7 @@ <h2 id="downloads">Downloads</h2>
   <li><a href="https://swcarpentry.github.io/shell-novice/shell-novice-data.zip"><code>shell-novice-data.zip</code></a></li>
   <!-- for swcarpentry/python-novice-inflammation -->
   <li><a href="https://swcarpentry.github.io/python-novice-inflammation/python-novice-inflammation-data.zip"><code>python-novice-inflammation-data.zip</code></a></li>
+  <!-- for swcarpentry/sql-novice-survey -->
+  <li><a href="https://files.software-carpentry.org/survey.db"><code>survey.db</code></a></li>
+  <li><a href="https://files.software-carpentry.org/survey.sql"><code>survey.sql</code></a></li>
 </ul>