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
Closes homebrew/linuxbrew#15