chore(.travis.yml): always check-out Lean source code
chore(.travis.yml): always check-out Lean source code

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