Add an attr_reader for 'head'
Add an attr_reader for 'head'

This allows `brew audit` to inspect the head URLs, for example; until
now, that check was failing silently.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

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