Use help-block class instead of help-text

[jgoux]

Hi,
is there a reason why you chose to use the "help-text" class in the bootstrap3ElementModifier ? If you use the "help-block" class instead, your message will be stylized by the "has-error" class applied on the formGroup element.
I'll hapilly make some PR if you want to.

[jonsamwell]

Not sure where I got help-text from - it does appear from the Bootstrap docs that 'help-block' is the correct class. It would be great if you do a PR - you might need to modify some of the test as well I think. Thanks!

[jonsamwell]

Fixed by b90f2d3