add travis-ci support to LXC github repo (v2)

add travis-ci support to LXC github repo (v2)

Travis is a free hosted CI platform for the open source community. It integrates
well with github and enables continous builds/tests for both repository itself
and all the pull requests so that one can quickly see the result of the possible
merge.

This yml file is one of the few required steps to enable travis-ci support for
LXC github repo. One of you guys still need to sign in travis-ci through GitHub OAuth
and enable travis support from its profile page https://travis-ci.org/profile

As an example https://travis-ci.org/caglar10ur/lxc-upstream/builds/15872074 can be seen

changes since v1;
 - All external dependencies are now innstalled via before_install section
 - Dropped all configure flags as Stéphane suggested

Signed-off-by: S.Çağlar Onur <caglar@10ur.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>