travis-ci: bumped the NGINX core to 1.17.4.

Author: thibaultcha

.travis.yml

@@ -35,7 +35,7 @@ env:
  - TEST_NGINX_SLEEP=0.006
  - LUACHECK_VER=0.21.1
  matrix:
- - NGINX_VERSION=1.17.1
+ - NGINX_VERSION=1.17.4
 
 before_install:
  - sudo luarocks install luacheck $LUACHECK_VER