#42: Fix incompatible mixin in navigation CSS

`border-radius` mixin is no longer supported by Bootstrap as of [TWBS
Pull 6342](“https://github.com/twbs/bootstrap/pull/6342):

>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