chore(.travis.yml): use ubuntu-14.04, g++-5, emacs25
chore(.travis.yml): use ubuntu-14.04, g++-5, emacs25

avigad/tutorial,Hodapp87/cs6460_project,avigad/tutorial,soonhokong/lean-tutorial,soonhokong/lean-tutorial,Hodapp87/cs6460_project,leanprover/tutorial,soonhokong/lean-tutorial,leanprover/tutorial,Hodapp87/cs6460_project,avigad/tutorial,leanprover/tutorial