Revert ".travis.yml: drop Werror build of xtp"
Revert ".travis.yml: drop Werror build of xtp"

This reverts commit 793fcc0fd76fe8e7d121cf9c92904fbc0f20de8c.

MrTheodor/csg,votca/csg,votca/csg,MrTheodor/csg,MrTheodor/csg,MrTheodor/csg,votca/csg,MrTheodor/csg,votca/csg