travis-ci: bumped the NGINX core to 1.19.3. (openresty#86)

Author: xiaocang

.travis.yml

@@ -25,7 +25,7 @@ env:
     - NGX_BUILD_JOBS=$JOBS
   matrix:
     - NGINX_VERSION=1.11.2
-    - NGINX_VERSION=1.17.8
+    - NGINX_VERSION=1.19.3
 
 services:
  - memcache