Add source_code_uri to gemspec

[cllns]

The rubygems page for hanami-cli doesn't link to (this) github repo.

There's soon-to-be-documented metadata fields that rubygems.org uses. The one we're adding here is source_code_uri. It adds a "Source Code" link to the rubygems.org page for the gem.

Example:
https://github.com/aws/aws-sdk-ruby/blob/master/gems/aws-sdk/aws-sdk.gemspec#L18
https://rubygems.org/gems/aws-sdk

We may want to add some of the other metadata fields that are available:

changelog_uri
documentation_uri
wiki_uri
mailing_list_uri
bug_tracker_uri
download_uri

But let's see how this works; it may be enough.
We should decide which we want, and apply the same changes to all Hanami repos. 🌸

(Thanks to @bkuhlmann for pointing this out)

[bkuhlmann]

@cllns Thanks, this is awesome!

[davydovanton]

👍