Testsuite fails with Minitest 5.20

[mtasaka]

As of clockwork git head ( 7ed9042 ),
testsuite fails with Minitest 5.20 as:

$ ruby -Ilib:test -rclockwork -e "Dir.glob('./test/**/*_test.rb').sort.each {|t| require t}"
Loaded suite -e
Started

  1) Error:
Clockwork::Manager#test_0012_exceptions are trapped and logged:
NameError: uninitialized constant MiniTest
    /builddir/build/GIT/clockwork/test/manager_test.rb:120:in `block (2 levels) in <top (required)>'

93 runs, 168 assertions, 0 failures, 1 errors, 0 skips

Already for a long while "Minitest" usage is preferred and with 5.19 "MiniTest" usage
is hidden by default.