Undo Bootstrap's normalisation of anchor box-sizing to avoid conflict

The styling of the schedule setting widget was being broken by Bootstrap's normalisation of the box-sizing attribute of elements. In [the Bootstrap docs](http://getbootstrap.com/getting-started/#third-parties) they cover this and show various options for fixing it.

As only anchor elements were being affected I only applied the fix to them—this fixes the issue for me when using Bootstrap 3, and a quick test without Bootstrap loaded shows no change from the default look.

The styling problem I observed looks to be the same or very similar to that of when you integrate it with Foundation (Issue https://github.com/GetJobber/recurring_select/issues/49) so perhaps this fix will also help there.