remove "node" (which resolves to 10) from travis

tomyam1 · tomyam1 · commit 11a724415cfb · 2018-05-01T18:14:04.000+03:00
It seems we need gulp 4 for it gulpjs/gulp#2162
diff --git a/.travis.yml b/.travis.yml
@@ -3,6 +3,5 @@ language: node_js
 node_js:
   - "6"
   - "8"
-  - "node"
 
 sudo: false
