travis.yml: use homebrew/homebrew-core on Linux.
travis.yml: use homebrew/homebrew-core on Linux.

sjackman/homebrew,EricFromCanada/brew,vitorgalvao/brew,JCount/brew,DomT4/brew,JCount/brew,vitorgalvao/brew,Linuxbrew/brew,maxim-belkin/brew,nandub/brew,sjackman/homebrew,reitermarkus/brew,MikeMcQuaid/brew,claui/brew,sjackman/homebrew,JCount/brew,mahori/brew,MikeMcQuaid/brew,sjackman/homebrew,Homebrew/brew,ilovezfs/brew,mahori/brew,hanxue/linuxbrew,DomT4/brew,claui/brew,reitermarkus/brew,Linuxbrew/brew,mgrimes/brew,MikeMcQuaid/brew,reitermarkus/brew,maxim-belkin/brew,konqui/brew,JCount/brew,EricFromCanada/brew,DomT4/brew,mgrimes/brew,DomT4/brew,EricFromCanada/brew,konqui/brew,Homebrew/brew,hanxue/linuxbrew,alyssais/brew,nandub/brew,nandub/brew,Homebrew/brew,maxim-belkin/brew,ilovezfs/brew,EricFromCanada/brew,vitorgalvao/brew,konqui/brew,mahori/brew,ilovezfs/brew,mgrimes/brew,mahori/brew,konqui/brew,vitorgalvao/brew,claui/brew,Homebrew/brew,MikeMcQuaid/brew,alyssais/brew,Linuxbrew/brew,nandub/brew,Linuxbrew/brew,reitermarkus/brew,alyssais/brew,claui/brew,hanxue/linuxbrew