Fix BamlDecompiler.Tests

Set VersionName = "rc"

Use MSBuildToolsPath instead of DevEnvDir.

Set VersionName = null

set VersionName = 'rc'

Set VersionName = "beta1"

Close icsharpcode#1033: Add TFM for net45

Push VSIX version number to 1.7.2

Tests: Set Roslyn language version to latest

Update to -beta4 NuGet, fix Workbook errors