devel/ruby-rspec-core: udpate to 3.9.1

Update ruby-rspec-core to 3.9.1.

### 3.9.1 / 2019-12-28
[Full Changelog](http://github.com/rspec/rspec-core/compare/v3.9.0...v3.9.1)

Bug Fixes:

* Prevent bisect command from blocking when number of specs exceeds file
  descriptor limit on OSX or Linux. (Benoit Tigeot, #2669)
* Prevent warnings being issued on Ruby 2.7.0. (Jon Rowe, #2680)

(taca)