luci-app-ddns: bump to version 2.4.0-1

luci-app-ddns: bump to version 2.4.0-1

- fixes for #611 and #620
- adaptions for ddns-scripts 2.6.0
-- add hint for problems with BusyBox nslookup compiled with musl and offer installing hostip package
-- add hint for not installed certificates inside /etc/ssl/certs and offer installing ca-certificates package https://github.com/openwrt/packages/pull/2243
-- check if malfunctional BusyBox nslookup installed, so disable using option dns_server if not Bind host or hostip installed
- optimizations
- cleanup: remove .po files without any translations

Signed-off-by: Christian Schoenebeck <christian.schoenebeck@gmail.com>