Switch to Minitest::Test instead of Test::Unit::TestCase

This gives us a consistent test framework across all Ruby versions which
should help avoid errors that are only found when CI runs the tests on
different Rubies. (And this fixes an immediate bug where there's no
`skip` method in the version of test-unit we're currently using only on
Ruby 2.2.)

test/test_language.rb

@@ -1,6 +1,6 @@
 require_relative "./helper"
 
-class TestLanguage < Test::Unit::TestCase
+class TestLanguage < Minitest::Test
   include Linguist
 
   def test_find_by_alias
@@ -198,7 +198,7 @@ class TestLanguage < Test::Unit::TestCase
   def test_find_all_by_extension
     Language.all.each do |language|
       language.extensions.each do |extension|
-        assert_include Language.find_by_extension(extension), language
+        assert_includes Language.find_by_extension(extension), language
       end
     end
   end
@@ -283,7 +283,7 @@ class TestLanguage < Test::Unit::TestCase
   end
 
   def test_error_without_name
-    assert_raise ArgumentError do
+    assert_raises ArgumentError do
       Language.new :name => nil
     end
   end