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: we specifically add a dependency on version 0.9.2.2 to support use of 'rake/rdoctask' for Ruby pre-1.9.3 builds
* Requires 'rdoc/task' for Ruby 1.9.3 and later instead of obsolete 'rake/rdoctask' in Rakefile
* Initializes Sequel DB object in test_helper.rb
* Enables the :after_initialize plugin for Sequel
* Restricts the required version of ActiveSupport to lower than 4.0.0 for Ruby pre-1.9.3