drop unsupported node versions from Travis config
diff --git a/.travis.yml b/.travis.yml
index 9c8593d..921e6c8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 sudo: false
 language: node_js
 node_js:
-  - "0.10"
-  - "0.12"
-  - iojs
   - "4"
+  - "6"
+  - "8"