Bootstrap is not loading for me. What am I doing wrong?

application.scss (complete file, nothing else in it)
----------------------------------------------------

```
// "bootstrap-sprockets" must be imported before "bootstrap" and "bootstrap/variables"
@import "bootstrap-sprockets";
@import "bootstrap";
@import "bootstrap/theme";

```

Gemfile
-------

```
gem 'bootstrap', '~> 4.0.0.alpha4'
gem 'bootstrap-sass', '~> 3.1.1'
gem 'bootstrap-sass-extras'

```

HTML IN HEADER SHOWS (running rails s, in development mode):
------------------------------------------------------------

```
<link rel="stylesheet" media="all" href="/stylesheets/application.css">

```