Update text to match actual requirements. ruby-prof/ruby-prof#303

Author: cfis

index.html

@@ -91,7 +91,7 @@ <h2><a id="installation">Installation</a></h2>
           installed so the extension can be built when it is installed. If you
           are running Windows, then you should install the Windows specific gem
           or install install <a href="https://rubyinstaller.org/add-ons/devkit.html">devkit</a>.</p>
-        <p>ruby-prof requires Ruby 2.4.0 or higher.</p>
+        <p>ruby-prof requires Ruby 2.5.0 or higher.</p>
         <p><strong>!!!WARNING!!!</strong></p>
         <p>Ruby versions 2.6.1, 2.6.2 and 2.6.3 have a <a href="https://bugs.ruby-lang.org/issues/15911">bug</a>
           that can cause ruby-prof to return incorrect results. </p>