@ishank I guess that the problem is caused by the following node-sass issue:
I would suggest you to downgrade to the latest Node v5.x.x - Node v5.11.0. Node v6.0.0 was just released and has a long list of breaking changes which might break some packages (as node-sass in this case). I guess that the upgrade to Node v6 will be safe in at least a month (or probably even 2-3 months) when the package authors manage to catch up with the breaking changes and update their packages to support Node v6.
Additionally, if you want to start a new Ionic 2 project then the correct syntax is (i.e. with --v2):
ionic start TestApp --v2