travis-ci: bumped the NGINX core to 1.21.4. (#242)

Author: xiaocang

.travis.yml

@@ -30,12 +30,12 @@ env:
     - OPENSSL_PREFIX=/opt/ssl
     - OPENSSL_LIB=$OPENSSL_PREFIX/lib
     - OPENSSL_INC=$OPENSSL_PREFIX/include
-    - OPENSSL_VER=1.1.1f
+    - OPENSSL_VER=1.1.1l
     - LD_LIBRARY_PATH=$LUAJIT_LIB:$LD_LIBRARY_PATH
     - TEST_NGINX_SLEEP=0.006
     - LUACHECK_VER=0.21.1
   jobs:
-    - NGINX_VERSION=1.19.3
+    - NGINX_VERSION=1.21.4
 
 before_install:
   - sudo luarocks install luacheck $LUACHECK_VER