tests: install specific Bundler version.
tests: install specific Bundler version.

Otherwise `brew tests` fails with the latest. See the failing
Homebrew/homebrew-test-bot `master` build as an example.

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