Created MessageBoard class and tests

Question: 
I have the latest version of RSpec installed and when I ran tests, it gave me the warning
    
    DEPRECATION: mock is deprecated. Use double instead.

Other sources said 'mock' was just an alias for 'double' anyways but I couldn't figure out how to use 'double' correctly. I suspect it has a slightly different function than mock as well. Can you give any guidance on that?