Change ProjectConfigurationPlatforms to be postSolution (#73)

When running SlnGen on a repository with a mix of .Net Standard and .Net framework the solution file generated would hang on load. After binary searching the changes I found that the solution file generated by Visual Studio has ProjectConfigurationPlatforms as postSolution. Making this one change causes the solution to properly load. 

Fixes #71

src/SlnGen.Build.Tasks/Internal/SlnFile.cs

@@ -164,7 +164,7 @@ public void Save(TextWriter writer, bool useFolders)
 
             writer.WriteLine("	EndGlobalSection");
 
-            writer.WriteLine("	GlobalSection(ProjectConfigurationPlatforms) = preSolution");
+            writer.WriteLine("	GlobalSection(ProjectConfigurationPlatforms) = postSolution");
             foreach (SlnProject project in _projects)
             {
                 foreach (string configuration in allConfigurations)
@@ -193,4 +193,4 @@ public void Save(TextWriter writer, bool useFolders)
             writer.WriteLine("EndGlobal");
         }
     }
-}
+}