Update remaining license headers to explicitly say GPLv2+

Also change ReadMe badge.
diff --git a/DateTimeMeter.h b/DateTimeMeter.h
index 04cc327..5196dd3 100644
--- a/DateTimeMeter.h
+++ b/DateTimeMeter.h
@@ -3,7 +3,7 @@
 /*
 htop - DateTimeMeter.h
 (C) 2004-2011 Hisham H. Muhammad
-Released under the GNU GPL, see the COPYING file
+Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */