Fix a few warnings, add GC control.

Created by: MrJoy

This fixes a couple Cucumber deprecation warnings, and a couple method redefinition warnings when run specs for rspec itself.

Also adds an option to explicitly constrain GC behavior allowing the user to minimize the number of GC calls during test runs which can trade memory for faster execution times. When this option is enabled, and configured such that one or more controlled GC runs then the GC time is subtracted from any relevant examples, and provided as a separate total when using --profile.