Use Current symlink instead of hardcode 1.8

Fixes issue #31966 

The #! should point at `Current`, which is a symlink to the "system" version of Ruby. This symlink exists by default on at least 10.6, 10.8, 10.9, and 10.10. In 10.10, Ruby 1.8 is not present, so the
install script fails to install brew on Yosemite beta (10.10).

While Yosemite is unreleased, the homebrew user community appears to have many early adopters who will upgrade to the beta, install it in VMs for testing, and buy new machines with it installed.