add type="button" to Bootstrap versions dropdown

[coliff]

minor accessibility issue fix reported by webhint.io browser extension.

[patrickhlauke]

ah, good spot. missing type="button" doesn't really have any adverse effect unless you're inside a <form> (as otherwise a button without type="button" automatically acts as a submit button, which may not be intended), but as a good practice, we should definitely have it there either way.