update license years

LICENSE.md

@@ -1,18 +1,18 @@
 Clustering.jl is licensed under the MIT License:
 
-> Copyright (c) 2012: all contributors to the package.
-> 
+> Copyright (c) 2012-2018: all contributors to the package.
+>
 > 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 the Software without restriction, including
 > without limitation the rights to use, copy, modify, merge, publish,
 > distribute, sublicense, and/or sell copies of the Software, and to
 > permit persons to whom the Software is furnished to do so, subject to
 > the following conditions:
-> 
+>
 > The above copyright notice and this permission notice shall be
 > included in all copies or substantial portions of the Software.
-> 
+>
 > THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 > EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 > MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND