Update LICENSE.txt for 2018.
The MIT License (MIT)
-Portions Copyright (c) 2015-2017, The PostgreSQL Global Development Group
-Portions Copyright (c) 2013-2017, David Steele
+Portions Copyright (c) 2015-2018, The PostgreSQL Global Development Group
+Portions Copyright (c) 2013-2018, David Steele
Permission is hereby granted, free of charge, to any person obtaining a copy of
this software and associated documentation files (the "Software"), to deal in
