Corrected copyright years (keras-team#9375)

Author: abrad1212

LICENSE

@@ -1,19 +1,19 @@
 COPYRIGHT
 
 All contributions by François Chollet:
-Copyright (c) 2015, François Chollet.
+Copyright (c) 2015 - 2018, François Chollet.
 All rights reserved.
 
 All contributions by Google:
-Copyright (c) 2015, Google, Inc.
+Copyright (c) 2015 - 2018, Google, Inc.
 All rights reserved.
 
 All contributions by Microsoft:
-Copyright (c) 2017, Microsoft, Inc.
+Copyright (c) 2017 - 2018, Microsoft, Inc.
 All rights reserved.
 
 All other contributions:
-Copyright (c) 2015 - 2017, the respective contributors.
+Copyright (c) 2015 - 2018, the respective contributors.
 All rights reserved.
 
 Each contributor holds copyright over their respective contributions.