precompile assets requires MAS_RAILS_SECRET_TOKEN

This is needed by devise which was previously not loaded as behind
feature toggles

assets:precompile rake task does not seem to set this variable

I'm hoping this will fix the build
Perhaps something like the following in [config/secrets.yml](https://github.com/moneyadviceservice/frontend/blob/cf1596023d95c2985a1c9ea6ab680b91fe0bc42f/config/secrets.yml#L8) could be useful to avoid `build.sh` from having to know too much about the internals of the app:
```ruby
secret_key_base: <%= ENV['MAS_RAILS_SECRET_TOKEN'] || 'blank' %>
```