Use rspec -P pattern option in rake tasks

rspec has an internal option (-P) to receive pattern of files to run via the command line, use this option in rake_tasks instead of passing through a huge list of files one by one on the command line.
When pattern is ./spec/... then rspec will just ignore it and not run any specs at all. Had to change it! :/

About the pattern, the docs for rspec command line in `lib/rspec/core/option_parser.rb` say that the default is `(default: "spec/**/*_spec.rb")`, while the actual default in `lib/rspec/core/configuration.rb` is `@pattern = '**/*_spec.rb'`.

I rather just fix the problem at hand, and leave the various defaults issue to another pull request.