add type attribute to button for bootstrap flashes

According to [bootlint](https://github.com/twbs/bootlint) all buttons should have a type shown [here](https://github.com/twbs/bootlint/blob/master/src/bootlint.js#L451-L456)

This pull request adds the default `type="button"` to the BootstrapFlashHelper.

References:
http://wtfhtmlcss.com/#buttons-type