* Update md5 sums for Ruby 2.4
Ruby 2.4 deprecated Fixnum & Bignum into Integer. This means the MD5 digests for the integers in our tests have a class of Integer instead of Fixnum which means we need to update the digests specifically for 2.4.
* Use Gem::Version for safer version comparison
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.)
