ci: test on Node.js 8.15 instead of 8.0.0

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - 8.0.0
+  - 8.15
   - 10
 git:
   depth: 5

appveyor.yml

@@ -1,6 +1,6 @@
 environment:
   matrix:
-    - nodejs_version: 8.0.0
+    - nodejs_version: 8.15
 install:
   - ps: Install-Product node $env:nodejs_version
   - npm set verify-store-integrity false