Replace SkipUnchangedFiles with a Condition

Author: bradwilson

src/xunit.runner.visualstudio/build/xunit.runner.visualstudio.targets

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Target Name="XunitVSTestCopyAbstractions" AfterTargets="Build">
-    <Copy SourceFiles="$(MSBuildThisFileDirectory)xunit.abstractions.dll" DestinationFolder="$(OutputPath)" SkipUnchangedFiles="true" />
+    <Copy Condition="!Exists('$(OutputPath)xunit.abstractions.dll')" SourceFiles="$(MSBuildThisFileDirectory)xunit.abstractions.dll" DestinationFolder="$(OutputPath)" />
   </Target>
 </Project>