This change updates OmniSharp with the latest MSBuild binaries. Recent performance changes for MSBuild now
require NuGet.Build.Tasks, which requires a set of NuGet binaries be included as well. Because the versions of these NuGet binaries might be different (actually, they *are* different) than the binaries that OmniSharp uses, there's a bit of duplication.

'BypassFrameworkInstallChecks' allows builds to skip a particularly hacky check in the GetReferenceAssemblyPaths
task. This hack affects projects that target a .NET Framework version less than 4.0. Essentially, the hack tries
to load a certain assembly from the GAC to "guarantee" that .NET Framework 3.5 SP1 is installed (an old case added
specifically for WPF projects). However, we don't include that assembly in our Mono package. So, many Unity projects
end up failing the check because they often target much early .NET Framework versions. We can fix that by just
skipping the check, which isn't really necessary when we're running on Mono.