updated dependencies

In light of the recent  SQL Injection Vulnerabilities Affecting PostgreSQL, which the current version of exercism is vulnerable to via activerecord ~> 4.0.4, I thought an update might be in order. This involved updating the  activerecord and activesupport  gems to 4.1.4. This also created a dependency with minitest 5.3.5 in the test/test_helper.rb file.

This created some issues with the test/api/stats_test.rb file. For some reason updating activerecord to the new version changed the date that was being rendered. I was able to apply a simple fix by explicitly stating what format was wanted. Minitest has now also started calling MiniTest::Unit::TestCase, MiniTest::Test; I have updated all of the test files accordingly and the tests all still pass. 

Please let me know if this pull request is ok as it is the first one I have ever submitted!