Update checkstyle (#82)

Update Grakn Core AGPL template, from 2018 to 2019.

Author: flyingsilverfin

checkstyle/config/checkstyle-file-header-agpl.txt

@@ -1,6 +1,6 @@
 ^\W*
 ^\W*GRAKN.AI - THE KNOWLEDGE GRAPH$
-^\W*Copyright \(C\) 2018 Grakn Labs Ltd$
+^\W*Copyright \(C\) 2019 Grakn Labs Ltd$
 ^\W*$
 ^\W*This program is free software: you can redistribute it and/or modify$
 ^\W*it under the terms of the GNU Affero General Public License as$