Lock to NS6 until NS7 becomes more stable

Author: agonper

.travis.yml

@@ -127,6 +127,6 @@ before_install:
   - sudo pip install six
 
 install:
-  - echo no | npm install -g nativescript
+  - echo no | npm install -g nativescript@6.5.0
   - tns usage-reporting disable
   - tns error-reporting disable