Use MiniTest::Unit::TestCase instead of Minitest::Test

Since Rails 4.0.x is depending on Minitest 4.x, the constant won't be
available so the gist won't run.

[ci skip]
Typo in Changelog.
Merge pull request #12164 from rywall/patch-1

Typo in Changelog. [ci skip]
[ci skip] Change link name of Rails i18n wiki.

Actually it's Ruby i18n link.
Merge pull request #12166 from JuanitoFatas/doc-patch-1

[ci skip] Change link name of Rails i18n wiki.
Merge pull request #12157 from robin850/patch-9

Use MiniTest::Unit::TestCase instead of Minitest::Test  [ci skip]