diagnostic: silence outdated Travis Xcode warning.
diagnostic: silence outdated Travis Xcode warning.

This doesn't add any value and stops us from testing `brew doctor` and
others from running `brew doctor` on Travis CI.

Homebrew/brew,muellermartin/dist,zmwangx/brew,sjackman/homebrew,pseudocody/brew,sjackman/homebrew,EricFromCanada/brew,jmsundar/brew,JCount/brew,EricFromCanada/brew,tonyg/homebrew,reelsense/brew,aw1621107/brew,Homebrew/brew,hanxue/linuxbrew,ilovezfs/brew,amar-laksh/brew_sudo,ilovezfs/brew,tonyg/homebrew,DomT4/brew,konqui/brew,aw1621107/brew,nandub/brew,vitorgalvao/brew,mahori/brew,DomT4/brew,MikeMcQuaid/brew,reitermarkus/brew,mgrimes/brew,konqui/brew,nandub/brew,MikeMcQuaid/brew,mahori/brew,JCount/brew,aw1621107/brew,MikeMcQuaid/brew,ilovezfs/brew,bfontaine/brew,rwhogg/brew,alyssais/brew,reitermarkus/brew,rwhogg/brew,nandub/brew,amar-laksh/brew_sudo,reitermarkus/brew,hanxue/linuxbrew,jmsundar/brew,EricFromCanada/brew,reelsense/brew,rwhogg/brew,palxex/brew,claui/brew,JCount/brew,maxim-belkin/brew,tonyg/homebrew,Linuxbrew/brew,DomT4/brew,amar-laksh/brew_sudo,bfontaine/brew,reelsense/brew,gregory-nisbet/brew,Linuxbrew/brew,konqui/brew,EricFromCanada/brew,zmwangx/brew,maxim-belkin/brew,maxim-belkin/brew,claui/brew,jmsundar/brew,mgrimes/brew,Homebrew/brew,pseudocody/brew,bfontaine/brew,nandub/brew,mgrimes/brew,Homebrew/brew,Linuxbrew/brew,reitermarkus/brew,konqui/brew,palxex/brew,vitorgalvao/brew,palxex/brew,muellermartin/dist,vitorgalvao/brew,gregory-nisbet/brew,gregory-nisbet/brew,claui/brew,vitorgalvao/brew,JCount/brew,MikeMcQuaid/brew,alyssais/brew,pseudocody/brew,mahori/brew,muellermartin/dist,DomT4/brew,zmwangx/brew,sjackman/homebrew,claui/brew,mahori/brew,hanxue/linuxbrew,Linuxbrew/brew,alyssais/brew,sjackman/homebrew