travis: bumped the nginx core version to 1.17.1.

Author: thibaultcha

.travis.yml

@@ -25,7 +25,7 @@ env:
     - NGX_BUILD_JOBS=$JOBS
   matrix:
     - NGINX_VERSION=1.11.2
-    - NGINX_VERSION=1.15.8
+    - NGINX_VERSION=1.17.1
 
 services:
  - memcache