change AC_PACKAGE_BUGREPORT from info@bitcoin.org to github issue tracker URL

Mentioned by wumpus:
http://bitcoinstats.com/irc/bitcoin-dev/logs/2015/05/02#l1430543664

Details here:
http://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.69/html_node/Initializing-configure.html

configure.ac

@@ -6,7 +6,7 @@ define(_CLIENT_VERSION_REVISION, 99)
 define(_CLIENT_VERSION_BUILD, 0)
 define(_CLIENT_VERSION_IS_RELEASE, false)
 define(_COPYRIGHT_YEAR, 2015)
-AC_INIT([Bitcoin Core],[_CLIENT_VERSION_MAJOR._CLIENT_VERSION_MINOR._CLIENT_VERSION_REVISION],[info@bitcoin.org],[bitcoin])
+AC_INIT([Bitcoin Core],[_CLIENT_VERSION_MAJOR._CLIENT_VERSION_MINOR._CLIENT_VERSION_REVISION],[https://github.com/bitcoin/bitcoin/issues],[bitcoin])
 AC_CONFIG_SRCDIR([src/main.cpp])
 AC_CONFIG_HEADERS([src/config/bitcoin-config.h])
 AC_CONFIG_AUX_DIR([build-aux])