travis: bumped the nginx core version to 1.17.1.

Author: thibaultcha

.travis.yml

@@ -17,7 +17,7 @@ env:
     - LD_LIBRARY_PATH=$LUAJIT_LIB:$LD_LIBRARY_PATH
   matrix:
     - NGINX_VERSION=1.9.15
-    - NGINX_VERSION=1.15.8
+    - NGINX_VERSION=1.17.1
 
 before_install:
   - sudo apt-get install -qq -y cpanminus libgd-dev