chore(travis): update node version (#904)

node@6.9.1 installs npm@3.10.8 by default and CLI@next fails to install on it. Issue: NativeScript/nativescript-cli#2980

Author: Vasil Chimev

.travis.yml

@@ -1,6 +1,6 @@
 env:
   global:
-  - NODE_VERSION=6.9.1
+  - NODE_VERSION=6.11.1
   - EMULATOR_API_LEVEL=19
   - ANDROID_VERSION=23
   - ANDROID_BUILD_TOOLS_VERSION=23.0.1