Add warning for memory measurement on OTP 18

[devonestes]

This way folks won't think things just aren't working, and will know to
upgrade their version of OTP to something newer.

Resolves #194

[PragTob]

Credo strikes again!

  Code Readability                                                              
┃ 
┃ [R] ↓ Do not use parentheses when defining a function which has no arguments.
┃       lib/benchee/configuration.ex:336 #(Benchee.Configuration.print_memory_measure_warning)
Please report incorrect results: https://github.com/rrrene/credo/issues
Analysis took 1.7 seconds (0.05s to load, 1.6s running checks)
331 mods/funs, found 1 code readability issue.
The command "mix credo --strict" exited with 4.

[PragTob]

Looks good save the credo error... I quickly thought if we were duplicating the logic but it's not the same so we're golden I think ⭐