Bump version to 0.4.0

docs/DataSources.html

@@ -23,7 +23,7 @@
         <a class="header-link" href="index.html">
           DiffableDataSources Docs
         </a>
-         (100% documented)
+         (96% documented)
       </p>
     
       <p class="header-col--secondary">
@@ -156,7 +156,7 @@ changes with automatic diffing.</p>
       </article>
     </div>
     <section class="footer">
-      <p>&copy; 2019 <a class="link" href="https://github.com/ra1028" target="_blank" rel="external">Ryo Aoyama</a>. All rights reserved. (Last updated: 2019-12-10)</p>
+      <p>&copy; 2020 <a class="link" href="https://github.com/ra1028" target="_blank" rel="external">Ryo Aoyama</a>. All rights reserved. (Last updated: 2020-04-12)</p>
       <p>Generated by <a class="link" href="https://github.com/realm/jazzy" target="_blank" rel="external">jazzy ♪♫ v0.11.2</a>, a <a class="link" href="https://realm.io" target="_blank" rel="external">Realm</a> project.</p>
     </section>
   </body>