downgrade ruby version dependency

### Downgrade ruby version dependency
In commit d9b59ff, `required_ruby_version` was changed from 1.8.6 to 1.9.3. However, I was able to successfully build and run the specs (`rake compile && rake test`) on ruby 1.8.7p358 (OSX 10.9.5). 

Likewise, `gem build libxslt-ruby.gemspec && gem install libxslt-ruby-1.1.1.gem` successfully installed the gem on ruby 1.8.7, although parsing documentation failed: rdoc/rdoc#327

### Update Rakefile
Additionally, the rake:rdoc task expects to find a README file. It instead needs to search for the README.rdoc file.