Gate 1.2 Beta installer to VS 2015 update 3 or later

[mjbvz]

NTVS 1.2 Beta will require VS 2015 Update 3 or later in order to pick up some important Typescript fixes.

We should gate the installer to this version, similar to what we did for the 1.2 Alpha and gating to VS 2015 update 2 or later: #806

The beta may actually have to be gated to the Update 3 RC, while the final release should be gated to Update 3 final

[mjbvz]

PR submitted #993 to handle Update 3 RC gating and opened issue to track gating of the final NTVS 1.2 release #994

[mjbvz]

Closing since we will only benefit by gating to the final release of VS 2015 Update 3, not the RC. This work is tracked by #994