update minitest gem to 5.3

Sync up with Homebrew.  Solves the following problem:
```bash
$ bundle exec rake test
... minitest/unit.rb:19:in `const_missing': uninitialized constant MiniTest::Test (NameError)
	from /usr/local/Library/Homebrew/test/testing_env.rb:91:in `<module:Homebrew>'
    ...
```
disable minitest-colorize

solves this error:
```bash
$ bundle exec rake test
...lib/minitest/colorize.rb:88:in `<top (required)>': undefined method `output=' for Minitest::Unit:Class (NoMethodError)
...
```
define global `shutup` method (temporary)

solves the following error:
```bash
$ bundle exec rake test
...
NoMethodError: undefined method `shutup'
...
```

This is not a permanent solution.  The test suite should be changed
around to find the new `shutup` method, which is still in Homebrew,
but no longer global.
Merge pull request #4982 from rolandwalker/test_env_changes

Adjust to Homebrew's test environment changes