Fix travis build

Kagami · Kagami · commit edeac0f31d1e · 2016-10-28T22:35:25.000+03:00
Latest gulp-sourcemaps requires ecma2015 and other stuff.
diff --git a/.travis.yml b/.travis.yml
@@ -1,8 +1,6 @@
 language: node_js
 node_js:
-  - "0.10"
-  - "0.12"
-  - "iojs"
+  - "node"
 sudo: false
 notifications:
   email:
