travis-ci: upgrade version number of nginx CORE, remove clang compiler mode from travis.

Author: jiahao

.travis.yml

@@ -25,7 +25,6 @@ cache:
 
 compiler:
   - gcc
-  - clang
 
 env:
   global:
@@ -40,7 +39,7 @@ env:
     - TEST_NGINX_SLEEP=0.006
   matrix:
     - NGINX_VERSION=1.11.2
-    - NGINX_VERSION=1.19.3
+    - NGINX_VERSION=1.19.9
 
 services:
  - mysql