drop node 4 test

ef4 · ef4 · commit fda7c2568662 · 2018-08-24T23:42:54.000-04:00
ember-cli itself has dropped support for node 4, and some dep is dying in node 4, so goodbye.
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,6 @@
 ---
 language: node_js
 node_js:
-  - "4"
   - "6"
   - "8"
 

-Original file line number
+Diff line change
@@ @@ -1,7 +1,6 @@ @@
 ---
 language: node_js
 node_js:
 -  - "4"
   - "6"
   - "8"