downgrade ruby version dependency

[sshao]

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: ruby/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.

[cfis]

Ruby 1.8.7 was end-of-lifed a year ago, so its no longer supported by this gem.

[sshao]

Are there technical reasons why it can't be supported? MRI 1.8.7 was EOL'd, but it's still being used (redhat continues to support 1.8.7 in RHEL), and Rubinius 1.3.4 (and as far as I know JRuby as well) both support 1.8.7; apps running on those could want to use libxslt-ruby.

[cfis]

There probably aren't any technical reasons, outside of its never tested anymore. JRuby wouldn't use libxslt-ruby since its a C extension, and no one has ever asked about Rubinius that I can recall.

So I just don't see any point to it.

[sshao]

This (in addition to my first PR) would actually allow a project I'm working on to use the latest version of libxslt-ruby on Rubinius 1.3.

[sshao]

Also, a passing Travis build on 1.8.7. I can't test this branch on rbx-1.3 right now because #11 causes the compilation step to fail on Travis, and I have to use rake compile:libxslt_ruby as the previously mentioned rdoc issue crops up in rake compile.