Run `brew` command with `--disable-gems` ruby option

I suppose Homebrew does not use external ruby gems. I this is the case -
Homebrew can run with `--disable-gems` option for ruby, which actually
makes `brew` a bit faster.

Without the patch:

    $ time brew -v
    Homebrew 0.9.5
    
    real  0m0.115s
    user  0m0.090s
    sys 0m0.022s

With the patch:

    $ time brew -v
    Homebrew 0.9.5
    
    real  0m0.096s
    user  0m0.073s
    sys 0m0.020s