update year in LICENSE, conf.py and README.md files

I found that year in files is not up-to-date

doc/conf.py

@@ -35,7 +35,7 @@ for mod_name in MOCK_MODULES:
 # General information about the project.
 project = u'xgboost'
 author = u'%s developers' % project
-copyright = u'2015, %s' % author
+copyright = u'2016, %s' % author
 github_doc_root = 'https://github.com/dmlc/xgboost/tree/master/doc/'
 
 # add markdown parser