Updated .travis.yml to use Node v10

no issue

- see 99c3338ca9 (diff-354f30a63fb0907d4ad57269548329e3)
- see https://docs.ghost.org/faq/node-versions/

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 sudo: false
 node_js:
-  - "8"
+  - "10"