fix(.travis.yml): push only if it's not trigger by a pull-request
fix(.travis.yml): push only if it's not trigger by a pull-request

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