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...
diff --git a/.travis.yml b/.travis.yml
index e2a32a2..809c229 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -102,7 +102,7 @@
     compiler: clang
     env: TARGET=linux-glibc FLAGS="USE_SLZ=1 USE_PCRE2=1 USE_PCRE2_JIT=1 USE_LUA=1 USE_OPENSSL=1 USE_SYSTEMD=1 USE_WURFL=1 WURFL_INC=contrib/wurfl WURFL_LIB=contrib/wurfl USE_51DEGREES=1" CC=clang-9
     before_script:
-      - git clone http://git.1wt.eu/git/libslz.git/
+      - git clone https://github.com/wtarreau/libslz
       - cd libslz && make && make PREFIX=${HOME}/opt install && cd ..
   allow_failures:
   - os: linux