Some fixes for current spree master

I was unable to run `bundle install` against current master.

```
Bundler could not find compatible versions for gem "sprockets":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets (~> 2.0.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.1) ruby depends on
          sprockets-rails (~> 2.0) ruby depends on
            sprockets (2.12.1)

Bundler could not find compatible versions for gem "sprockets-rails":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      sprockets-rails (~> 2.0.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.1) ruby depends on
          sprockets-rails (2.1.3)

Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    sass-rails (>= 0) ruby depends on
      railties (~> 3.1.0) ruby

    spree (>= 0) ruby depends on
      spree_core (= 2.3.0.beta) ruby depends on
        rails (~> 4.1.1) ruby depends on
          railties (4.1.1)
```

Also fix the font-awesome `sass/rails` issue which was also in `spree-product-assembly`

Also fix a spec which was failing due to the a refunded payment's source being the original payment.

Remind me again why we're locking down the sprockets version here, please.