Add build dir to ignore list

Signed-off-by: Philip Luyckx <philip.luyckx@gmail.com>
Implement https://github.com/pluyckx/kam/issues/3

Copy old config file to /etc/kam/kam.conf.back when installing a new
version.

Signed-off-by: Philip Luyckx <philip.luyckx@gmail.com>