Run browsers tests with Node.js 8 rather than Node.js 6

Because standard dev dependencies being installed for not-legacy versions,
has introduced some features or syntax constructs that simply won't
run for Node.js 6 and below.

Therefor running render-and-run-in-browser tests with Node.js 8 instead.

.travis.yml

@@ -1,12 +1,11 @@
 language: node_js
 node_js:
-  - 6
   - 8
   - 10
   - 12
 script:
   - npm test
-  - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '6' || npm run test-browser"
+  - "test $TRAVIS_PULL_REQUEST != 'false' || test $TRAVIS_NODE_VERSION != '8' || npm run test-browser"
 env:
   global:
   - secure: L0dg0jr2fwkc2tPwP5svybILPBn2qdLzMrWc5tEXg3MPcy8D59Gvf+ri7INqo+ETPM20o5CsaDCH+LHUNS/V0G4VG1ajvsy7d8uh3hnb/K6VfVui/CjsHIqOcOZrbxVxgyX+iMXEXAj0+Syow9uDQHVhrz1qqad1n79likNCXa4=