#42: Fix incompatible mixin in navigation CSS

`border-radius` mixin is no longer supported by Bootstrap as of <a
href=“https://github.com/twbs/bootstrap/pull/6342” target=“_blank”>TWBS
Pull 6342: </a>
>Remove .border-radius() and .border-*-*-radius mixins.
>As only Android 2.1, iOS 3.2, and older desktop browsers require a
prefixed version, we've
>removed the base mixin. Since we no longer require prefixes for
independent corners, we've
>dropped those mixins as well. Mixins for a single side, like
.border-left-radius, are still available.
So I just deleted the references in components/navigations.css.scss and
added that file back to the import list in application.css.scss