FIxing path separator to address #247 (#248)

* Correcting path separator to ;

* Using the proper API from System.IO to get path separator

src/Microsoft.AspNetCore.NodeServices/HostingModels/OutOfProcessNodeInstance.cs

@@ -126,7 +126,7 @@ public void Dispose()
             var existingNodePath = Environment.GetEnvironmentVariable("NODE_PATH") ?? string.Empty;
             if (existingNodePath != string.Empty)
             {
-                existingNodePath += ":";
+                existingNodePath += Path.PathSeparator;
             }
 
             var nodePathValue = existingNodePath + Path.Combine(projectPath, "node_modules");