.travis.yml: only build on latest OS X. (#74)
.travis.yml: only build on latest OS X. (#74)

We have Jenkins to test the other versions.
bfontaine/brew,nandub/brew,JCount/brew,MikeMcQuaid/brew,claui/brew,zmwangx/brew,vitorgalvao/brew,alyssais/brew,EricFromCanada/brew,claui/brew,aw1621107/brew,hanxue/linuxbrew,mgrimes/brew,AnastasiaSulyagina/brew,Homebrew/brew,konqui/brew,vitorgalvao/brew,bfontaine/brew,gregory-nisbet/brew,JCount/brew,DomT4/brew,nandub/brew,ilovezfs/brew,mahori/brew,mgrimes/brew,jmsundar/brew,aw1621107/brew,nandub/brew,nandub/brew,MikeMcQuaid/brew,reelsense/brew,gregory-nisbet/brew,reelsense/brew,pseudocody/brew,palxex/brew,EricFromCanada/brew,mistydemeo/homebrew,zmwangx/brew,mahori/brew,JCount/brew,rwhogg/brew,palxex/brew,muellermartin/dist,reelsense/brew,Linuxbrew/brew,staticfloat/brew,tonyg/homebrew,vitorgalvao/brew,mgrimes/brew,konqui/brew,EricFromCanada/brew,claui/brew,konqui/brew,Homebrew/brew,jmsundar/brew,maxim-belkin/brew,hanxue/linuxbrew,tonyg/homebrew,mistydemeo/homebrew,aw1621107/brew,hanxue/linuxbrew,DomT4/brew,Homebrew/brew,konqui/brew,gordonmcshane/homebrew,gregory-nisbet/brew,AnastasiaSulyagina/brew,sjackman/homebrew,ilovezfs/brew,mahori/brew,vitorgalvao/brew,mahori/brew,staticfloat/brew,palxex/brew,reitermarkus/brew,jmsundar/brew,toonetown/homebrew,Homebrew/brew,gordonmcshane/homebrew,toonetown/homebrew,pseudocody/brew,amar-laksh/brew_sudo,reitermarkus/brew,ilovezfs/brew,toonetown/homebrew,sjackman/homebrew,amar-laksh/brew_sudo,JCount/brew,sjackman/homebrew,muellermartin/dist,maxim-belkin/brew,DomT4/brew,Linuxbrew/brew,alyssais/brew,maxim-belkin/brew,tonyg/homebrew,zmwangx/brew,rwhogg/brew,sjackman/homebrew,EricFromCanada/brew,bfontaine/brew,Linuxbrew/brew,mistydemeo/homebrew,amar-laksh/brew_sudo,DomT4/brew,reitermarkus/brew,staticfloat/brew,claui/brew,Linuxbrew/brew,gordonmcshane/homebrew,alyssais/brew,rwhogg/brew,MikeMcQuaid/brew,pseudocody/brew,reitermarkus/brew,AnastasiaSulyagina/brew,MikeMcQuaid/brew,muellermartin/dist