don't require test/unit for compatibility with minitest 5.0

  cucumber required test/unit in `disable_mini_and_test_unit_autorun.rb`.
This breaks compatibility with minitest >= 5.0 and leads to the error described in seattlerb/minitest#283

there's an example that demonstrates the incompatibility at https://gist.github.com/levinalex/772817a7d453addf4b91

Why did the `require "test/unit"` exist in the fist place? Removing it does not seem to break any tests.