Don't check issues on Ruby 1.8.6

The system certs are too old to be able to read from Github, so
don't try to use the Github API on 1.8.6 either.

Library/Homebrew/exceptions.rb

@@ -219,7 +219,7 @@ class BuildError < Homebrew::InstallationError
       end
     end
     puts
-    unless RUBY_VERSION < "1.8.6" || issues.empty?
+    unless RUBY_VERSION < "1.8.7" || issues.empty?
       puts "These open issues may also help:"
       puts issues.map{ |i| "#{i['title']} (#{i['html_url']})" }.join("\n")
     end