Use a longer timeout on SSH multiplex sockets to github.com

When running SSH without an existing socket a new instance is forked
holding open stderr, this causes Composer to hang waiting for output
from that stream. Increase the timeout so there is less chance of
running Composer when there is no socket open.

See: composer/composer#4180

.ssh/hosts.d/github.com/config

@@ -1,2 +1,3 @@
 User git
 RequestTTY no
+ControlPersist 2h