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.
[Relevant](https://github.com/rspec/rspec-core/commit/20ed0c6e894b5f73b2db2e788b09fae158667a1a)