Officially support minitest v4.5.0.

Note that I'm planning on releasing this as a 0.12.x patch release so
people with earlier versions of ActiveSupport can use a fairly recent
version of Mocha with the latest version of MiniTest. At the moment,
earlier versions (e.g. 3.0.x, 3.1.x, 3.2.x) of ActiveSupport do not
work with 0.13.x versions of Mocha.
Bump version to 0.12.9 and prepare for release.