Update to Minitest 5

[strzibny]

Hi,

this is the first attempt to update the test suite to Minitest 5. We are going to upgrade Ruby on Rails in Fedora 21 and RoR 4.1 uses the new Minitest so we upgraded it as well. We would love to see gems to use the new Minitest not to carry patches or sticking with the old one.

What this PR changes?

• require 'test/unit' to require 'minitest/autorun'
• Test::Unit::TestCase to Minitest::Test
• assert_not_nil to refute_nil
• adds requirement of Minitest 5 to .gemspec
• adds assert_nothing_raised, assert_not_same, and assert_block to other custom asserts extending Kernel in test/test_utils.rb. This is kind of backport from older Minitest and we can probably change the tests not to use those at all in future.

Please consider.

[philr]

Thanks for submitting this pull request.

I have merged the change.