Use MiniTest::Test

I've been getting this deprecation warning when using `MiniTest::Unit::TestCase` on `ruby 2.0.0p195`.

```
MiniTest::Unit::TestCase is now Minitest::Test. From sourcemaps/test/test_map.rb:4:in `<top (required)>'
MiniTest::Unit::TestCase is now Minitest::Test. From sourcemaps/test/test_offset.rb:4:in `<top (required)>'
MiniTest::Unit::TestCase is now Minitest::Test. From sourcemaps/test/test_vlq.rb:4:in `<top (required)>'
```