Update for iperf-3.6.

RELEASE_NOTES

@@ -1,4 +1,4 @@
-== iperf 3.6 2018-06-?? ==
+== iperf 3.6 2018-06-25 ==
 
 * Notable user-visible changes
 

configure.ac

@@ -24,7 +24,7 @@
 # file for complete information.
 
 # Initialize the autoconf system for the specified tool, version and mailing list
-AC_INIT(iperf, 3.5+, https://github.com/esnet/iperf, iperf, https://software.es.net/iperf/)
+AC_INIT(iperf, 3.6, https://github.com/esnet/iperf, iperf, https://software.es.net/iperf/)
 m4_include([config/ax_check_openssl.m4])
 AC_LANG(C)