Update to Minitest 5

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.