MiniTest -> Minitest

`MiniTest` was changed to `Minitest` in 5.0.0:

> Renamed MiniTest to Minitest. Your pinkies will thank me. (aliased to MiniTest)

In 5.19.0 the following was changed:

> Only load minitest/unit (aka ancient MiniTest compatibility layer) if ENV

So test broke under >= 5.19.0.

test/sparoid_test.rb

@@ -59,7 +59,7 @@ def test_it_sends_message_with_empty_cache_file
   end
 
   def test_it_resolves_public_ip_only_once_per_instance
-    dns = MiniTest::Mock.new
+    dns = Minitest::Mock.new
     dns.expect :getresource, Resolv::IPv4.create("1.1.1.1"), ["myip.opendns.com", Resolv::DNS::Resource::IN::A]
     Resolv::DNS.stub(:open, ->(_, &blk) { blk.call dns }) do
       s = Sparoid::Instance.new