Replaced outdated node versions 0.11, 0.12, 4, 5 in travis build with…

… 6, 7, 8

.travis.yml

@@ -1,9 +1,8 @@
 language: node_js
 node_js:
-  - "0.11"
-  - "0.12"
-  - "4"
-  - "5"
+  - "6"
+  - "7"
+  - "8"
 env:
   - CXX=g++-4.8
 addons:
@@ -19,9 +18,9 @@ matrix:
   fast_finish: true
   include:
     - os: linux
-      node_js: "4"
+      node_js: "6"
       env: CXX=g++-4.8 TEST_SUITE=coveralls
     - os: linux
-      node_js: "4"
+      node_js: "6"
       env: CXX=g++-4.8 TEST_SUITE=lint
 script: npm run $TEST_SUITE