Upgrade to Spire published version

[denisrosset]

Right now, we use a -SNAPSHOT using a Git submodule, remove this when the Spire GCD PRs go through.

[denisrosset]

Have to update:

• [x] remove the git submodule
• [x] update the contributors instructions
• [x] update the build.sbt

[denisrosset]

Done!