chore(ci): use node 6.9.1 for travis-ci build

This is an attempt at fixing the failing e2e tests.

For more details on failure see: https://travis-ci.org/esvit/ng-table/builds/178617133

.travis.yml

@@ -1,7 +1,7 @@
 sudo: false
 language: node_js
 node_js:
-- node
+- 6.9.1
 cache:
   directories:
   - node_modules
@@ -32,4 +32,4 @@ after_success:
 
 branches:
   except:
-    - "/^v\\d+\\.\\d+\\.\\d+$/"
+    - "/^v\\d+\\.\\d+\\.\\d+$/"