travis: enable password authentication on OSX

Our ssh tests assume that the server supports password authentication
in a few places. This is convenient as we're not testing authentication
methods, but what happens around them.

Tell sshd on OSX to accept this form of authentication.

script/cibuild.sh

@@ -22,7 +22,13 @@ ctest -V . || exit $?
 # can do the push tests over it
 
 killall git-daemon
-sudo start ssh
+
+if [ "$TRAVIS_OS_NAME" = "osx" ]; then
+    echo 'PasswordAuthentication yes' | sudo tee -a /etc/sshd_config
+else
+    sudo start ssh
+fi
+
 ssh-keygen -t rsa -f ~/.ssh/id_rsa -N "" -q
 cat ~/.ssh/id_rsa.pub >>~/.ssh/authorized_keys
 ssh-keyscan -t rsa localhost >>~/.ssh/known_hosts