Fix a few more rspec deprecations hiding out in the sunspot test suite.

* mock -> double
* raise_error
Add rake to the sunspot gemfile, mainly to satisfy travis ci.

https://travis-ci.org/sunspot/sunspot/jobs/10522726
Fix the sunspot and sunspot_rails doc task under 1.8.7, primarily to get travis ci green.
correct Sunspot.remove! with block test, and get it to pass by propagating block down the call chain.
allow all #remove_by_id(!) to accept (splat or) array of ids, and add testing coverage
Use rake db:test:prepare in sunspot_rails test setup.