Copyrights: update for 2015

And by "update", I mean "drop occurrences of the year entirely",
so that we don't have to deal with this every 12 months.

LICENSE.md

 The MIT License
 
-Copyright (c) 2011-2014 LibGit2Sharp contributors
+Copyright (c) LibGit2Sharp contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

LibGit2Sharp/Properties/AssemblyInfo.cs

@@ -17,7 +17,7 @@
 #endif
 
 [assembly: AssemblyProduct("LibGit2Sharp")]
-[assembly: AssemblyCopyright("Copyright © 2011-2014 LibGit2Sharp contributors")]
+[assembly: AssemblyCopyright("Copyright © LibGit2Sharp contributors")]
 
 [assembly: CLSCompliant(true)]