Fix for the submodule inclusion

Hello,

The problem this PR solves is the issue @teneightfive experienced when using your gem:

the files of the submodule of the gem weren't being included. 

This can be proved by the following steps:

```
$ git clone git@github.com:alphagov/govuk_frontend_toolkit_gem.git
$ cd govuk_frontend_toolkit_gem
$ gem build govuk_frontend_toolkit.gemspec 
$ mv govuk_frontend_toolkit-0.39.0.gem del
$ cd del
$ tar xvf govuk_frontend_toolkit-0.39.0.gem
x metadata.gz
x data.tar.gz
x checksums.yaml.gz
$ tar zxvf data.tar.gz 
x .gitignore
x .gitmodules
x CONTRIBUTING.md
x Gemfile
x LICENCE
x README.md
x Rakefile
x govuk_frontend_toolkit.gemspec
x jenkins.sh
x lib/govuk_frontend_toolkit.rb
x lib/govuk_frontend_toolkit/engine.rb
x lib/govuk_frontend_toolkit/version.rb
```
at this point you should not see any files under **app/assets**.

Due to this we've experienced build failures with our app. Please see:

https://travis-ci.org/ministryofjustice/opg-lpa/builds/16535855

(it's failing since it can't find **colours.scss** file, see line *1212*).

Once our patch was added: 

https://github.com/ministryofjustice/opg-lpa/commit/d65b317f849919f2a8ed8017d60e99d774d69117

the build passed:

https://travis-ci.org/ministryofjustice/opg-lpa/builds/16581101

Please consider merging this into your gem.