version.c: Update copyright year to range 2002-2015.

version.c: Update copyright year to range 2002-2015.

Although we don't update the year in each source file any more,
version.c contains the text that is printed with --version. So this
text should be up to date, and it should be a range. This commit
changes it from "2014" to "2002-2015".

2002 seems appropriate for the first year in the range since the
initial cvs commit was in 2002. Some code is older than that, but
that's probably not important.