The first release in a long time: Ruby 2.4 support and plenty of fixes

Thanks @lostapathy for motivating the release. 👍

Enhancements

• Ruby 2.4 support #56
• Don't define errors descending from ArgumentError rather than Exception #58
• Return the default value when an attribute is present but empty #60
• Remove support for Rails < 4

Bug fixes

• fix errors in namespace writing #52
• namespaceify all elements of an xpath, rather than just the first #49
• fix the :else option to also work when creating a new object #44
• fix array default reader to always return the same object #46

Miscellaneous

• Switch from jeweler to juwelier
• Switch from test-unit to minitest
• Upgrade to rspec > 3 and modern rspec syntax
• Remove references to the defunct rubyforge.org