CI: travis-ci: fix libslz download URL

let us switch to github mirror, travis cannot download original libslz

error is:
fatal: unable to access 'http://git.1wt.eu/git/libslz.git/': Failed
to connect to git.1wt.eu port 80: Connection timed out The command
"git clone http://git.1wt.eu/git/libslz.git/" failed and exited with
128...
1 file changed