tests: revert branch name of lua-ngixn-module-plus in .travis.yml. (#54)

Author: zhuizhuhaomeng

.travis.yml

@@ -69,7 +69,7 @@ install:
   - git clone -b plus [email protected]:orinc/openresty-devel-utils-plus.git ../openresty-devel-utils-plus
   - git clone https://github.com/openresty/resty-cli.git ../resty-cli
   - git clone https://github.com/simpl/ngx_devel_kit.git ../ndk-nginx-module
-  - git clone -b keepalive [email protected]:orinc/lua-nginx-module-plus.git ../lua-nginx-module
+  - git clone -b plus [email protected]:orinc/lua-nginx-module-plus.git ../lua-nginx-module
   - git clone https://github.com/openresty/no-pool-nginx.git ../no-pool-nginx
   - git clone https://github.com/openresty/echo-nginx-module.git ../echo-nginx-module
   - git clone https://github.com/openresty/lua-resty-lrucache.git