Depend on BuildTime packages

Author: AArnott

src/ImmutableObjectGraph.Generation.NuGet/ImmutableObjectGraph.Generation.nuspec

@@ -12,7 +12,7 @@
     <tags></tags>
     <dependencies>
       <group targetFramework=".NETPortable0.0-net45+win+wpa81+wp80+MonoAndroid10+xamarinios10+MonoTouch10">
-        <dependency id="CodeGeneration.Roslyn" version="0.1.54" />
+        <dependency id="CodeGeneration.Roslyn.BuildTime" version="0.1.57" />
       </group>
     </dependencies>
   </metadata>