Gate NTVS 1.2 Alpha 2015 to VS 2015 Update 2+

[mjbvz]

Currently you can install NTVS 1.2 Alpha on older version of VS 2015 just fine, but many features will not work as expected. Specifically, ES6 Intellisense is pretty broken before Update 2.

[mjbvz]

I think we want to do something similar to what PTVS is doing: https://github.com/Microsoft/PTVS/blob/d3786c8b0e83c5db3001af303126f1f8cd8af576/Common/Setup/LaunchConditions.wxs#L62

I'm not familiar with the installer file format, but I think just adding a check for VS14 with a max version check to VS_PRERELEASE of pre "Update 2" would prevent new installs.

[mjbvz]

Update 2 seems to be devenv.exe 14.0.25123.0, and, although the documentation is rather lacking, I believe MaxVersion is inclusive, so the check would be:

<FileSearch Id="VSDevEnvFileVersionSearch" Name="devenv.exe" MinVersion="14.0.0.0" MaxVersion="14.0.25122.0"/>