Bump upper end of range to 2020.
These copyright notices are user-facing; they're visible in several
dialogs in Eclipse. It is strange or even misleading to see a copyright
notice for JGit saying "2005, 2010" when there have been *many*
developments in the past ten years.
Change-Id: Idaa6244b2b3d9cecb29cc690085f8d008195cf12
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>