Improve detection of app-local IDE MSBuild for dev15
Improve detection of app-local IDE MSBuild for dev15

In addition to detecting the VsInstallRoot property, we also need
to check the VisualStudioVersion being built for, since an extension
author could be using MSBuild 15.0 to build an extension that targets
14.0 or prior. So we make the selection of app-local paths depend
on the version being greater or equal to 15.0. Prior versions will
default to the registry-provided envvars for locating DevEnvDir
and Public/PrivateAssemblies

clariuslabs/VisualStudio