Remove node 6 from travis

Author: Paul Korzhyk

.travis.yml

@@ -1,12 +1,11 @@
 language: node_js
 node_js:
-  - "node"
-  - "8"
-  - "6"
+    - "node"
+    - "8"
 sudo: required
 dist: trusty
 before_install:
-  - scripts/install_dgraph.sh
+    - scripts/install_dgraph.sh
 script:
-  - scripts/build.sh
+    - scripts/build.sh
 after_success: npm run coveralls