versions: fix bottle_filenames.
versions: fix bottle_filenames.

Previously if a bottle had no checksum for a particular platform (i.e.
currently there are no Mavericks bottles) then it would generate a
different revision (and thus filename) to the rest of the bottles being
generated which meant bottles needed to be manually renamed.

Instead check the actual bottle object's checksums to make sure that
we've looked at the previous bottles for all platforms rather than just
the current one.
Linuxbrew/brew,MikeMcQuaid/brew,hanxue/linuxbrew,reitermarkus/brew,palxex/brew,toonetown/homebrew,rwhogg/brew,hanxue/linuxbrew,muellermartin/dist,sjackman/homebrew,maxim-belkin/brew,Homebrew/brew,bfontaine/brew,reitermarkus/brew,Homebrew/brew,claui/brew,konqui/brew,vitorgalvao/brew,MikeMcQuaid/brew,rwhogg/brew,konqui/brew,EricFromCanada/brew,JCount/brew,ilovezfs/brew,JCount/brew,staticfloat/brew,mgrimes/brew,sjackman/homebrew,gordonmcshane/homebrew,alyssais/brew,aw1621107/brew,muellermartin/dist,Homebrew/brew,staticfloat/brew,konqui/brew,reitermarkus/brew,mistydemeo/homebrew,rwhogg/brew,zmwangx/brew,nandub/brew,amar-laksh/brew_sudo,konqui/brew,claui/brew,zmwangx/brew,mgrimes/brew,Linuxbrew/brew,reitermarkus/brew,mistydemeo/homebrew,amar-laksh/brew_sudo,JCount/brew,aw1621107/brew,palxex/brew,mistydemeo/homebrew,palxex/brew,vitorgalvao/brew,tonyg/homebrew,ilovezfs/brew,AnastasiaSulyagina/brew,pseudocody/brew,DomT4/brew,gordonmcshane/homebrew,reelsense/brew,staticfloat/brew,maxim-belkin/brew,gordonmcshane/homebrew,zmwangx/brew,Linuxbrew/brew,jmsundar/brew,DomT4/brew,vitorgalvao/brew,sjackman/homebrew,AnastasiaSulyagina/brew,mahori/brew,claui/brew,ilovezfs/brew,tonyg/homebrew,MikeMcQuaid/brew,toonetown/homebrew,DomT4/brew,nandub/brew,nandub/brew,EricFromCanada/brew,muellermartin/dist,claui/brew,bfontaine/brew,alyssais/brew,reelsense/brew,pseudocody/brew,mahori/brew,JCount/brew,hanxue/linuxbrew,tonyg/homebrew,nandub/brew,alyssais/brew,DomT4/brew,jmsundar/brew,toonetown/homebrew,Linuxbrew/brew,bfontaine/brew,gregory-nisbet/brew,AnastasiaSulyagina/brew,mahori/brew,jmsundar/brew,gregory-nisbet/brew,gregory-nisbet/brew,amar-laksh/brew_sudo,EricFromCanada/brew,aw1621107/brew,EricFromCanada/brew,MikeMcQuaid/brew,reelsense/brew,sjackman/homebrew,mgrimes/brew,vitorgalvao/brew,Homebrew/brew,pseudocody/brew,mahori/brew,maxim-belkin/brew