formula_installer: fix default formula prune.
formula_installer: fix default formula prune.

Only prune it if it's not already installed.

Fixes bug with https://github.com/Homebrew/brew/pull/3479
Closes https://github.com/Homebrew/brew/pull/3535.

vitorgalvao/brew,JCount/brew,reitermarkus/brew,aw1621107/brew,JCount/brew,Homebrew/brew,konqui/brew,konqui/brew,MikeMcQuaid/brew,vitorgalvao/brew,DomT4/brew,sjackman/homebrew,Homebrew/brew,reitermarkus/brew,vitorgalvao/brew,pseudocody/brew,MikeMcQuaid/brew,JCount/brew,claui/brew,claui/brew,nandub/brew,mahori/brew,nandub/brew,hanxue/linuxbrew,Linuxbrew/brew,hanxue/linuxbrew,ilovezfs/brew,JCount/brew,Linuxbrew/brew,sjackman/homebrew,EricFromCanada/brew,EricFromCanada/brew,mgrimes/brew,alyssais/brew,alyssais/brew,reelsense/brew,maxim-belkin/brew,maxim-belkin/brew,reelsense/brew,konqui/brew,reelsense/brew,aw1621107/brew,mahori/brew,pseudocody/brew,mahori/brew,mgrimes/brew,hanxue/linuxbrew,vitorgalvao/brew,reitermarkus/brew,claui/brew,claui/brew,sjackman/homebrew,sjackman/homebrew,ilovezfs/brew,DomT4/brew,DomT4/brew,mahori/brew,EricFromCanada/brew,alyssais/brew,reitermarkus/brew,Homebrew/brew,Linuxbrew/brew,pseudocody/brew,EricFromCanada/brew,konqui/brew,MikeMcQuaid/brew,Linuxbrew/brew,maxim-belkin/brew,nandub/brew,Homebrew/brew,aw1621107/brew,ilovezfs/brew,nandub/brew,DomT4/brew,MikeMcQuaid/brew,mgrimes/brew