Update .exe's project.json Target Framework from dnxcore50 to netstandardapp1.5.
Update .dll's project.json Target Framework from dnxcore50 to netstandard1.3.
Adding workaround for DataContractSerialization to src\dotnet\project.json to fix crossgen issue.
Build 23901 has a dependency issue that doesn't allow the runtime.any.System.Private.DataContractSerialization
package to be restored. When we move to a new build of CoreFX we should take this workaround out.
DOTNET_HOME is no longer required, though it is a documented override, so this change removes all unnecessary references to DOTNET_HOME from the CLI Repo.
