Correcting compilation for the last lxc development version.

Hi,  

[This commit](https://github.com/lxc/ruby-lxc/commit/d7d3a0aed168be92652b6ef2fb69fe9da4eefd95) breaks compilation with master branch of [lxc](https://github.com/lxc/lxc) : the bdev_specs struct has been added to the lxccontainer.h.

I propose to add a new branch that will be dedicated to keep compatibility with the lxc version in ubuntu repository.