Use class applicationName variable instead of local.

I keep getting an exception from Path.Combine after updating to 0.99 if applicationName parameter is null, as suggested since it's now optional.

I guess this is an oversight and the local applicationName is used to find the rootAppDirectory, even though the class member applicationName is correctly set beforehand.

src/Squirrel/UpdateManager.cs

@@ -41,11 +41,11 @@ public UpdateManager(string urlOrPath,
             this.urlDownloader = urlDownloader ?? new FileDownloader();
 
             if (rootDirectory != null) {
-                this.rootAppDirectory = Path.Combine(rootDirectory, applicationName);
+                this.rootAppDirectory = Path.Combine(rootDirectory, this.applicationName);
                 return;
             }
 
-            this.rootAppDirectory = Path.Combine(rootDirectory ?? GetLocalAppDataDirectory(), applicationName);
+            this.rootAppDirectory = Path.Combine(rootDirectory ?? GetLocalAppDataDirectory(), this.applicationName);
         }
 
         public async Task<UpdateInfo> CheckForUpdate(bool ignoreDeltaUpdates = false, Action<int> progress = null)
@@ -273,4 +273,4 @@ static string getUpdateExe()
             return target.FullName;
         }
     }
-}
+}