[Fix] Libgen engine

Libgen has switched to new domain (i.e https://libgen.is) with TLS
support and older domain (i.e. http://libgen.io) is no longer
accessible. See, https://en.wikipedia.org/wiki/Library_Genesis, for more
information.

Resolves: #1693

searx/settings.yml

@@ -407,7 +407,7 @@ engines:
 
   - name : library genesis
     engine : xpath
-    search_url : http://libgen.io/search.php?req={query}
+    search_url : https://libgen.is/search.php?req={query}
     url_xpath : //a[contains(@href,"bookfi.net")]/@href
     title_xpath : //a[contains(@href,"book/")]/text()[1]
     content_xpath : //td/a[1][contains(@href,"=author")]/text()