fix(.travis.yml): do upgrade to fix 'Unable to correct problems, you have held broken packages' problem
fix(.travis.yml): do upgrade to fix 'Unable to correct problems, you have held broken packages' problem

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