don't run travis on old node versions

The test suite uses some syntax that isn't supported in Node 5.
Could change it but it doesn't seem like there is much of an advantage.

Author: alexkrolick

.travis.yml

@@ -2,8 +2,6 @@ language: node_js
 
 node_js:
   - "node"
-  - "6"
-  - "5"
 
 before_install:
   - npm install -g npm
@@ -14,4 +12,4 @@ install:
 script:
   - make build
   - make spec
-  - make lint
+  - make lint