Bump up version number to 1.21.0

configure.ac

@@ -2,7 +2,7 @@
 # Process this file with autoconf to produce a configure script.
 #
 AC_PREREQ([2.67])
-AC_INIT([aria2],[1.20.0],[https://github.com/tatsuhiro-t/aria2/issues],[aria2],[https://aria2.github.io/])
+AC_INIT([aria2],[1.21.0],[https://github.com/tatsuhiro-t/aria2/issues],[aria2],[https://aria2.github.io/])
 
 AC_CANONICAL_HOST
 AC_CANONICAL_TARGET