Travis CI: Create specific build for kernel 4.15.x (Ubuntu 18.04 and derived distros)

.travis.yml

@@ -74,3 +74,13 @@ matrix:
             - gcc-7
             - libssl1.1
       env: COMPILER=gcc-7 KVER=4.19.67
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+            - sourceline: 'ppa:ondrej/nginx-mainline'
+          packages:
+            - gcc-7
+            - libssl1.1
+      env: COMPILER=gcc-7 KVER=4.15.18