Tests: hard-code minitest < 5.0.0 until 5.0.0 compatibility issues can b...

Latest minitest release causes test failures. Use last known version for now until failures can be evaluated in detail:

d:\sources\OSS\cloud\deltacloud-core\server>rake test:drivers:mock
Warning: you should require 'minitest/autorun' instead.
From D:/Programs/Ruby193/lib/ruby/1.9.1/test/unit.rb:3:in `require'
  D:/Programs/Ruby193/lib/ruby/1.9.1/test/unit.rb:3:in `<top
(required)>'
  D:/Programs/Ruby193/bin/testrb:2:in `require'
  D:/Programs/Ruby193/bin/testrb:2:in `<main>'
MiniTest::Unit::TestCase is now Minitest::Test. From
D:/Programs/Ruby193/lib/ruby/1.9.1/test/unit/testcase.rb:8:in
`<module:Unit>'
D:/Programs/Ruby193/lib/ruby/1.9.1/test/unit.rb:328:in `autorun':
uninitialized class variable @@installed_at_exit in Test::Unit::Runner
(NameError)
        from D:/Programs/Ruby193/lib/ruby/1.9.1/test/unit.rb:640:in
`<top (required)>'
        from D:/Programs/Ruby193/bin/testrb:2:in `require'
        from D:/Programs/Ruby193/bin/testrb:2:in `<main>'
rake aborted!

This is with ruby 1.9.3p385 (2013-02-06) [i386-mingw32].