Prefer node 8 for single run tests in Travis CI

node@8 will be faster than node@6 at running all these tests,
and will offer better compatibility as our dependencies update and stop
supporting older LTS versions

.travis.yml

@@ -23,12 +23,11 @@ before_install:
 before_script:
   # these build targets only need to run once per build, so let's conserve a few resources
   # ESLint only supports Node >=4
-  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run lint; fi
-  # run these on node 6 to have npm 3 with flat dependencies
-  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run lint-markdown; fi
-  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run test-headless; fi
-  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ]; then npm run test-webworker; fi
-  - if [ "x$TRAVIS_NODE_VERSION" = "x6" ] && [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then npm run test-cloud; fi
+  - if [ "x$TRAVIS_NODE_VERSION" = "x8" ]; then npm run lint; fi
+  - if [ "x$TRAVIS_NODE_VERSION" = "x8" ]; then npm run lint-markdown; fi
+  - if [ "x$TRAVIS_NODE_VERSION" = "x8" ]; then npm run test-headless; fi
+  - if [ "x$TRAVIS_NODE_VERSION" = "x8" ]; then npm run test-webworker; fi
+  - if [ "x$TRAVIS_NODE_VERSION" = "x8" ] && [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then npm run test-cloud; fi
 
 script:
   - npm run test-node