Use Current symlink instead of hardcode 1.8

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).

Signed-off-by: Jack Nagel <jacknagel@gmail.com>