travis: only build on macOS.
travis: only build on macOS.

We have Azure Pipelines doing Linux builds for us now. They also do
macOS builds but CodeCov doesn't work there (yet).

mahori/brew,EricFromCanada/brew,sjackman/homebrew,Linuxbrew/brew,nandub/brew,konqui/brew,Homebrew/brew,vitorgalvao/brew,nandub/brew,JCount/brew,claui/brew,vitorgalvao/brew,reitermarkus/brew,mahori/brew,sjackman/homebrew,MikeMcQuaid/brew,MikeMcQuaid/brew,vitorgalvao/brew,EricFromCanada/brew,reitermarkus/brew,reitermarkus/brew,konqui/brew,JCount/brew,claui/brew,mahori/brew,sjackman/homebrew,vitorgalvao/brew,sjackman/homebrew,EricFromCanada/brew,Linuxbrew/brew,nandub/brew,claui/brew,konqui/brew,MikeMcQuaid/brew,Linuxbrew/brew,Homebrew/brew,nandub/brew,Homebrew/brew,claui/brew,MikeMcQuaid/brew,JCount/brew,Homebrew/brew,mahori/brew,Linuxbrew/brew,JCount/brew,konqui/brew,reitermarkus/brew,EricFromCanada/brew