Fixes to unit tests and updates to configuration to support CI builds using Travis CI

* Adds .travis.yml configuration file with entries for Ruby 1.8.7, 1.9.3 and 2.0.0
* Updates Gemfile to add dependency on "rake" required by Travis CI
* Requires 'rdoc/task' instead of obsolete 'rake/rdoctask' in Rakefile
* Initialize Sequel DB object in test_helper.rb
* Enable the :after_initialize plugin for Sequel