use `./` for all relative path imports (play nice with atomify/npm-less)

I'm trying to use bootstrap with [atomify](https://github.com/atomify/atomify), and if I try to import bootstrap's less files like this...

````css
@import 'bootstrap/less/bootstrap.less';
````

...it fails because [npm-less](https://github.com/Raynos/npm-less/pull/5) gets "confused" and interprets the file names as module names.

This patch in bootstrap solves the problem while allowing `npm-less` to keep a more consistent behavior.