Create a new commit which updates the gem.version field in hiera-vault.gemspec.
Then:
git commit -m "vx.y.z release"
git push
git tag vx.y.z
git push --tags
The new tagged version should be released automatically.
Now build the gem:
gem build hiera-vault.gemspec
And push it to rubygems.org, it will prompt for credentials if necessary:
gem push hiera-vault-x.y.z.gem