CI: test-unit-ruby-core fails on Ruby 2.4 - syntax error, unexpected keyword_rescue, expecting keyword_end

This Issue is to say that the Ruby 2.4 fails its tests, due to test-unit-ruby-core using newer Ruby syntax.

```
2.4.0/gems/test-unit-ruby-core-1.0.15/lib/core_assertions.rb:390: syntax error, unexpected keyword_rescue, expecting keyword_end (SyntaxError)
 rescue => marshal_error
```