title: Ruby Float#round behaviour change after update
body: <p>I am upgrading a repository from Ruby version 2.3.3 to 2.5.1. A test is failing, and I have narrowed down the cause to the following behaviour:</p>
  
  <p><code>In version 2.3.3
  1.34875.round(4)
  =&gt; 1.3487</code></p>
  
  <p><code>In version 2.5.1
  1.34875.round(4)
  =&gt; 1.3488</code></p>
  
  <p>Now, I'm aware of <a href="https://bugs.ruby-lang.org/issues/12958" rel="nofollow noreferrer">this change</a>, but I don't think it's relevant because a) the default behaviour was left alone, and b) the observed change is opposite to the proposed change in the default. I'm also aware that floating point numbers are not a good way to accurately store finite decimals, and that some change in precision might explain why this change has occurred. But I don't know, and I don't know how to find out.  </p>