Allow .well-known URI for letsencrypt

See https://letsencrypt.readthedocs.org/en/latest/using.html#webroot

.htaccess

@@ -42,6 +42,7 @@
   RewriteRule ^\.well-known/caldav /remote.php/dav/ [R=301,L]
   RewriteRule ^remote/(.*) remote.php [QSA,L]
   RewriteRule ^(build|tests|config|lib|3rdparty|templates)/.* - [R=404,L]
+  RewriteCond %{REQUEST_URI} !^/.well-known/acme-challenge/.*
   RewriteRule ^(\.|autotest|occ|issue|indie|db_|console).* - [R=404,L]
 
   # Rewrite rules for `front_controller_active`