test(gitbook-treeview): set testing environment with node 8

Author: aleen42

.travis.yml

@@ -1,15 +1,6 @@
 language: node_js
 node_js:
-  - "5"
-  - "5.1"
-  - "4"
-  - "4.2"
-  - "4.1"
-  - "4.0"
-  - "0.12"
-  - "0.11"
-  - "0.10"
-  - "iojs"
+  - "8"
 script: npm test
 os:
   - linux