Default infer_base_class_for_anonymous_controllers to true

For RSpec 3 this option should now default to true. Updated
the config value setting and the spec_helper generator template
to instead indicate the default value with the line to set it
commented out.

Fixes #869
Merge branch 'infer-anonymous-controller-base-class-by-default' of https://github.com/thomas-holmes/rspec-rails into thomas-holmes-infer-anonymous-controller-base-class-by-default
Adds a bit more clarifying details
Removes setting from default spec_helper.rb
Merge branch 'thomas-holmes-infer-anonymous-controller-base-class-by-default'
Changelog for #869, #893