Add travis integration

Though travis does not support lxc, still this will enable us building the gem against lxc daily build and across multiple versions of ruby. This PR add travis config to build ruby-lxc across ruby 1.9, 2.0 and 2.1 . Once integrated (we have to manually authorize travis service against the repo) , we'll be able to hyperlink travis build status/details inside the README file. Sample build details can be found [here](https://travis-ci.org/ranjib/ruby-lxc/builds/16029172)