Update .travis.yml

Author: xxczaki

.travis.yml

@@ -15,9 +15,9 @@ env:
 matrix:
   fast_finish: true
   include:
-    - node_js: '8'
+    - node_js: '9'
       env: TARGET=test-node COVERAGE=true
-    - node_js: '7'
+    - node_js: '8'
       env: TARGET=test-node
     - node_js: '6'
       env: TARGET=test-node