Fix compatibility with Minitest 5.19+ (#114)

The `MiniTest` was renamed to `Minitest`:

minitest/minitest@9a57c52

And the `MiniTest` constant is now loaded just when `MT_COMPAT` environment variable is set:

minitest/minitest@a2c6c18

This fixes following issue:

~~~
$ ruby -Ilib:test -e 'Dir.glob "./test/**/*_test.rb", &method(:require)'
/builddir/build/BUILD/test/test_helper.rb:17:in `<top (required)>': uninitialized constant MiniTest (NameError)
class MiniTest::Test
      ^^^^^^^^
Did you mean?  Minitest
	from <internal:/usr/share/rubygems/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/usr/share/rubygems/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from /builddir/build/BUILD/image_processing-1.12.2/usr/share/gems/gems/image_processing-1.12.2/test/builder_test.rb:1:in `<top (required)>'
	from <internal:/usr/share/rubygems/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:/usr/share/rubygems/rubygems/core_ext/kernel_require.rb>:85:in `require'
	from <internal:dir>:220:in `glob'
	from -e:1:in `<main>'
~~~

test/test_helper.rb

@@ -14,7 +14,7 @@
 require "vips"
 require "mini_magick"
 
-class MiniTest::Test
+class Minitest::Test
   def fixture_image(name)
     File.open("test/fixtures/#{name}", "rb")
   end