fix: classes are updated to be C# 6 compatible (#8)

### Changed
- Code updated to be C# 6 compatible

Author: srcnalt

Runtime/Interfaces/IResponse.cs

@@ -2,6 +2,6 @@ namespace OpenAI
 {
     public interface IResponse
     {
-        public ApiError Error { get; set; }
+        ApiError Error { get; set; }
     }
 }

Runtime/OpenAIApi.cs

@@ -15,8 +15,20 @@ public class OpenAIApi
         ///     Remember that your API key is a secret! Do not share it with others or expose it in any client-side code (browsers, apps).
         ///     Production requests must be routed through your own backend server where your API key can be securely loaded from an environment variable or key management service.
         /// </summary>
-        private Configuration configuration; 
-        private Configuration Configuration => configuration ??= new Configuration();
+        private Configuration configuration;
+
+        private Configuration Configuration
+        {
+            get
+            {
+                if (configuration == null)
+                {
+                    configuration = new Configuration();
+                }
+
+                return configuration;
+            }
+        }
 
         /// OpenAI API base path for requests.
         private const string BASE_PATH = "https://api.openai.com/v1";