dispose of the DbCommand used to execute migrations after we've finished with it rather than within the loop

disposing of it within the loop causes Mono.Data.Sqlite.dll to get upset, and it's the wrong behaviour anyway

OpenSim/Data/Migration.cs

@@ -149,8 +149,9 @@ namespace OpenSim.Data
                     UpdateVersion(_type, newversion);
                 }
                 version = newversion;
-                cmd.Dispose();
             }
+
+            cmd.Dispose();            
         }
 
         // private int MaxVersion()