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

Since we officially support Ruby 1.8.7, 1.9.3 and 2.0.0, it is important to regularly (and automatically) test against all three platforms. This change adds a Travis CI configuration file for all three versions of Ruby and fixes up a few files to include the correct versions of our dependencies:

* 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"
* 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 1.8.7 (CI builds only)

Please refer to the following location for recent build logs of the CI running against my fork of this repository:

https://travis-ci.org/rcook/attr_encrypted

You'll see now that we have 100% passing against all three versions of Ruby. I think it would be great to accept this pull request and add shuber/attr_encrypted to Travis so we can see regular build updates etc.