README: add ruby 1.8.6 requirement

For now, it seems that homebrew-core requires Ruby 1.8.6

Known limitations of Ruby 1.8.5:
* RUBY_PATCHLEVEL is not defined
* instance_variable_defined? is not defined
* pathname.rb:223 - 'Digest::SHA2' => 'Digest::SHA256' is necessary

closes homebrew/linuxbrew#12

I will look into it a little bit more to see if it is worthwile supporting older versions of Ruby, but it was pointed out in #12 that supporting older versions (e.g. tigerbrew) leads to limitations in how homebrew functions. I think it's reasonable to have this requirement for now.

(this is a fixed commit message from homebrew/linuxbrew#14)