fix: remove node 6 support

Signed-off-by: BoHong Li <a60814billy@gmail.com>

Author: a60814billy

.travis.yml

@@ -8,12 +8,6 @@ env:
 
 jobs:
   include:
-    - env: task=npm-test
-      node_js:
-        - 6
-      before_install:
-        - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version "$YARN_VERSION"
-        - export PATH="$HOME/.yarn/bin:$PATH"
     - env: task=npm-test
       node_js:
         - 8