Update ruby-minitest to 5.9.1.

=== 5.9.1 / 2016-09-25

* 2 bug fixes:

  * Re-release to refresh gem certificate signing. ugh.
  * Fixed hoe/minitest to not augment load path if we're actually testing minitest.

=== 5.9.0 / 2016-05-16

* 8 minor enhancements:

  * Added Minitest.info_signal accessors to customize signal for test run info. (nate)
  * Added assert_mock to make it more clear that you're testing w/ them.
  * Added negative filter by test name. (utilum)
  * Added warning to README that 1.8 and 1.9 support will be dropped in minitest 6.
  * Automatically activate minitest/hell if $MT_HELL is defined.
  * Improved default error messages for assert and refute. (bhenderson)
  * minitest/hell now tries to require minitest/proveit
  * mu_pp for strings prints out non-standard encodings to improve assert_equal diffs.

* 1 bug fix:

  * Removed Interrupt from PASSTHROUGH_EXCEPTIONS (already handled). (waldyr)

(taca)