Align Version button with Save / Cancel on front-end edit.

Fix for issue #4637

If you have versioning on, then a front end editing page has the version button misaligned with the Save and Cancel buttons.

This is due to the Save and Cancel buttons being rendered with the `<button>` tag whilst Version is rendered with the `<a>` tag - and different CSS is applied.

![image](https://cloud.githubusercontent.com/assets/3001893/4431400/ff5eb44c-4665-11e4-9168-5324af80b1e8.png)

The fix is to change the `<a>` tag for a `<button>` tag in `/libraries/cms/form/field/contenthistory.php`  at lines 49/53.