travis-ci: bumped the NGINX core to 1.21.4. (#278)

Author: xiaocang

.travis.yml

@@ -39,8 +39,8 @@ env:
     - NGX_BUILD_JOBS=$JOBS
     - TEST_NGINX_SLEEP=0.006
   matrix:
-    - NGINX_VERSION=1.21.3 OPENSSL_VER=1.1.0l
-    - NGINX_VERSION=1.21.3 OPENSSL_VER=1.1.1l
+    - NGINX_VERSION=1.21.4 OPENSSL_VER=1.1.0l
+    - NGINX_VERSION=1.21.4 OPENSSL_VER=1.1.1l
 
 services:
  - memcache