test-bot: avoid duplicate coverage reports
test-bot: avoid duplicate coverage reports

When running on Travis CI, both the Linux and macOS build will send a
coverage report, causing them to be merged by Coveralls. This results
in inferior coverage due to the early stage of the Linux-specific tests
and is probably not what we want. Make sure we only send a report for
macOS (assuming we stick with a single macOS build in `.travis.yml`).

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