doc: update to current V8 versions

Update the documentation to the correct V8 versions
in the guide *Maintaining V8 in Node.js*.

PR-URL: nodejs/node#11787
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/guides/maintaining-V8.md

@@ -93,11 +93,11 @@ includes the following branches<sup>1</sup>:
    </td>
    <td>2017-04-01
    </td>
-   <td>5.4
+   <td>5.5
    </td>
-   <td>2016-10-18
+   <td>2016-12-06
    </td>
-   <td>~2016-12-01
+   <td>2017-01-24
    </td>
   </tr>
   <tr>
@@ -107,11 +107,11 @@ includes the following branches<sup>1</sup>:
    </td>
    <td>N/A
    </td>
-   <td>5.4
+   <td>5.6
    </td>
-   <td>2016-10-18
+   <td>2017-01-31
    </td>
-   <td>~2016-12-01
+   <td>2017-03-07
    </td>
   </tr>
 </table>