Update to 1.815
    
    - New upstream release 1.815
      - Fixed divide-by-zero error in MCE->yield
      - Refactored code for the interval option by moving the code to the manager
        process, which allows the manager process to accomodate the next available
        worker ready to run; previously, a worker taking a long time resulted in
        empty time slots
      - Revised the description of posix_exit in MCE::Core.pod