ci: use node versions 8, 10, 11 for travis

Author: s1na

.travis.yml

@@ -1,8 +1,8 @@
 language: node_js
 node_js:
-  - "6"
   - "8"
-  - "9"
+  - "10"
+  - "11"
 env:
   - CXX=g++-4.8
 addons:
@@ -18,9 +18,9 @@ matrix:
   fast_finish: true
   include:
     - os: linux
-      node_js: "6"
+      node_js: "8"
       env: CXX=g++-4.8 TEST_SUITE=coveralls
     - os: linux
-      node_js: "6"
+      node_js: "8"
       env: CXX=g++-4.8 TEST_SUITE=lint
 script: npm run $TEST_SUITE