Fix compatibility with liblxc 4.0.4 and newer

[aither64]

ruby-lxc is broken with LXC 4.0.4 and newer, as two used external functions were removed from liblxc (see #45). This PR makes ruby-lxc work again with the following changes:

• lxc_config_parse_arch() from liblxc was used in LXC.arch_to_personality(). I don't know what can this method be used for, I've just removed it.
• lxc_wait_for_pid_status() does nothing special, I've added it's implementation into ruby-lxc.

Fixes #45