update travis configuration to current LTS node versions

Author: sokra

.travis.yml

@@ -1,17 +1,6 @@
 sudo: false
 language: node_js
 node_js:
-  - "0.10"
-  - "0.11"
-  - "0.12"
-  - "iojs-1"
-  - "iojs-2"
-  - "iojs-3"
   - "4"
-matrix:
-  allow_failures:
-    - node_js:
-      - "0.11"
-      - "iojs-1"
-      - "iojs-2"
-      - "iojs-3"
+  - "6"
+  - "8"