memcached listens on 'localhost' (ipv4 and ipv6)

Some client libraries (at least Ruby's 'dalli')
tried to connect to localhost, which resolved
to IPv6 first, which wasn't listening.

Listening to 'localhost' seems to cover anything
listed in /etc/hosts.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/memcached.rb

@@ -49,7 +49,7 @@ Add "-d" to start it as a daemon.
   <array>
     <string>#{HOMEBREW_PREFIX}/bin/memcached</string>
     <string>-l</string>
-    <string>127.0.0.1</string>
+    <string>localhost</string>
   </array>
   <key>RunAtLoad</key>
   <true/>