Travis CI: Specify distro version.
Travis CI: Specify distro version.

rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang,rgolubtsov/nonlinear-optimization-algorithms-multilang