Changed the border-color of .navbar-form to @navbar-default-border

The difference is 0.5% darkness since the old value was `darken(@navbar-default-bg, 7%)` and the value of `@navbar-default-border` is `darken(@navbar-default-bg, 6.5%)`

I hit on this one when I tried to change the color of the `navbar-default-bg` to `transparent` and after I fixed all the related ones in the `variables.less` there was a problem in the `navbar.less`.