Change required ruby version 1.9.3 -> 1.8.7

Semi-reverts part of commit d9b59ff.
Updated rake:rdoc task.

Changed "README" -> "README.rdoc".
Add rdoc and rake to gemspec dev dependencies
Add .travis.yml for testing on MRI 1.8.7
Add Gemfile for travis to install dependencies
Merge pull request #12 from sshao/1.1.1/downgrade_ruby_version_dependency

downgrade ruby version dependency