Removed dependency on bootstrap-sass

The dependency in the `forem.gemspec` file was preventing me from pulling the `3` branch of bootstrap-sass - I noticed it was already removed in `master` (in [this commit](https://github.com/radar/forem/commit/41095c8d1a816d272e440e9e47f3c5530e32252c)), so this PR is meant to match this.