Fix spelling of "Minitest".

This PR fixes spelling errors in `README.txt`, `Rakefile` and `design_rationale.rb`:
`MiniTest` -> `Minitest`.

Note: I didn't touch extensions' descriptions on purpose.

During the review I stumbled upon `.autotest` which might need some :heart:. The class mappings seem to be outdated. Do you want me to fix this in a separate PR?