add matrix gem, which is required for builds and no longer in ruby stdlib

[colinxfleming]

I rule and have completed some work on Case Manager that's ready for review!

Heroku builds are failing with:

/tmp/codon/tmp/buildpacks/50d5eddf222a9b7326028041d4e6509f915ccf2c/lib/language_pack/helpers/rake_runner.rb:100:in `load_rake_tasks!': Could not detect rake tasks (LanguagePack::Helpers::RakeRunner::CannotLoadRakefileError)
ensure you can run `$ bundle exec rake -P` against your app
and using the production group of your Gemfile.
rake aborted!
LoadError: cannot load such file -- matrix

turns out matrix got separated from the ruby stdlib but our heroku builds depend on it still, so re-adding it to our gemfile explicitly.

see gdelugre/origami#81 for example

This pull request makes the following changes:

• add matrix gem to gemfile

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For reviewer:

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[colinxfleming]

godmode to get deployable