Remove hanna as a development dependency.

It's long-unmaintained.
Rely on RDoc::Task rather than the defunct Rake::RDocTask
Fix the sunspot and sunspot_rails doc tasks by referencing the proper relative path to the readme file.
Fix the sunspot and sunspot_rails doc task under 1.8.7, primarily to get travis ci green.
Note that rails 4.0 is tested in the sunspot_rails readme.
Add rake to the sunspot gemfile, mainly to satisfy travis ci.

https://travis-ci.org/sunspot/sunspot/jobs/10522726
Use rake db:test:prepare in sunspot_rails test setup.
If we can't load rdoc, don't provide the doc rake tasks.
Merge pull request #495 from sourcery/rdoc-travis-updates

Rdoc travis updates