travis: bumped the nginx core version to 1.15.8.

Author: thibaultcha

.travis.yml

@@ -48,8 +48,8 @@ env:
     - DRIZZLE_VER=2011.07.21
     - TEST_NGINX_SLEEP=0.006
   matrix:
-    - NGINX_VERSION=1.13.6 OPENSSL_VER=1.0.2q OPENSSL_OPT="" OPENSSL_PATCH_VER=1.0.2h
-    - NGINX_VERSION=1.13.6 OPENSSL_VER=1.1.0j OPENSSL_OPT="" OPENSSL_PATCH_VER=1.1.0d
+    - NGINX_VERSION=1.15.8 OPENSSL_VER=1.0.2q OPENSSL_OPT="" OPENSSL_PATCH_VER=1.0.2h
+    - NGINX_VERSION=1.15.8 OPENSSL_VER=1.1.0j OPENSSL_OPT="" OPENSSL_PATCH_VER=1.1.0d
     # TODO: when adding an OpenSSL version >= 1.1.1, please add "enable-tls1_3"
     # to $OPENSSL_OPT.