utils/gems: fix finding bundler.
utils/gems: fix finding bundler.

Need to specify the executable name not variable here.

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