Updated Travis CI configuration
Use gemspec and bundle/gem_tasks instead of Jeweler

This commit introduces changes in order to use the standard way of
dealing with rubygems -- having a gemspec in the repository plus using
bundle/gem_tasks to manage building and uploading the gem to rubygems.
Update Rakefile

- Remove Jeweler.
- Add console task to open IRB with statsample-glm loaded.
Merge pull request #1 from agarie/use_bundler_gem_tasks

Use bundler gem tasks