Require Node 8+, also test PHP 7.2 and 7.3

.travis.yml

@@ -2,12 +2,14 @@ language: php
 sudo: false
 dist: trusty
 php:
+  - 7.3
+  - 7.2
   - 7.1
   - nightly
 
 env:
-  - TRAVIS_NODE_VERSION="6"
-  - TRAVIS_NODE_VERSION="7"
+  - TRAVIS_NODE_VERSION="8"
+  - TRAVIS_NODE_VERSION="10"
 
 matrix:
   fast_finish: true