chore(.travis.yml): build Lean only if TEST is ON
chore(.travis.yml): build Lean only if TEST is ON

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