use new firefly image

Author: dongweiming

firefly/static/pics/dongxi.jpg

@@ -58,7 +58,7 @@
               div.title
                 a href='/'
                   span class='.valign-helper'
-                  img.site-logo.logo-big src="${url_for('static', filename='pics/dongxi.jpg')}" alt='pythoncn'
+                  img.site-logo.logo-big src="${url_for('static', filename='pics/firefly.jpg')}" alt='pythoncn'
               div.panel
                 - if current_user.is_authenticated()
                   Hi ${current_user.username if current_user.username else current_user.cn}