feat(.travis.yml): move to container-based infrastructure
feat(.travis.yml): move to container-based infrastructure

See: http://docs.travis-ci.com/user/migrating-from-legacy/?utm_source=legacy-notice&utm_medium=banner&utm_campaign=legacy-upgrade

leanprover/tutorial,fpvandoorn/tutorial,soonhokong/lean-tutorial,avigad/tutorial,Hodapp87/cs6460_project,leanprover/tutorial,avigad/tutorial,fpvandoorn/tutorial,cjmazey/fork_leanprover_tutorial,leanprover/tutorial,cjmazey/fork_leanprover_tutorial,avigad/tutorial,Hodapp87/cs6460_project,Hodapp87/cs6460_project,soonhokong/lean-tutorial,cjmazey/fork_leanprover_tutorial,fpvandoorn/tutorial,soonhokong/lean-tutorial