travis-ci: bumped the NGINX core to 1.19.3. (#64)

Author: xiaocang

.travis.yml

@@ -36,7 +36,7 @@ env:
  - NGX_BUILD_JOBS=$JOBS
  matrix:
  - NGINX_VERSION=1.9.15
- - NGINX_VERSION=1.17.8
+ - NGINX_VERSION=1.19.3
 
 services:
  - redis-server