Don't release tar.gz, just tar.xz

I think it's quite safe to drop tar.gz these days and just distribute tar.xz. What do you all think? @jvcelak @dsalzman @dtaborsky ?

configure.ac

@@ -2,7 +2,7 @@
AC_PREREQ([2.60])
AC_INIT([knot], [2.0.0-beta], [knot-dns@labs.nic.cz])
-AM_INIT_AUTOMAKE([gnits subdir-objects dist-xz -Wall -Werror])
+AM_INIT_AUTOMAKE([gnits subdir-objects no-dist-gzip dist-xz -Wall -Werror])
AM_SILENT_RULES([yes])
AC_CONFIG_SRCDIR([src/knot/main.c])
AC_CONFIG_HEADERS([src/config.h])