data/ui: Update about dialog copyright dates

You wouldn't know it, but turns out I've done work on this since 2013.

Author: kaiw

data/ui/application.ui

@@ -10,7 +10,7 @@
     <property name="type_hint">dialog</property>
     <property name="program_name">Meld</property>
     <property name="copyright" translatable="yes">Copyright © 2002-2009 Stephen Kennedy
-Copyright © 2009-2013 Kai Willadsen</property>
+Copyright © 2009-2017 Kai Willadsen</property>
     <property name="website">http://meldmerge.org/</property>
     <property name="website_label" translatable="yes">Website</property>
     <property name="authors">Stephen Kennedy