Add gem-release support

[kennyadsl]

This PR introduces gem-release to release this extension with ease.

Supposing you are on the master branch and you are working on a fork of this extension, upstream is the main remote and you have write access to it, you can simply run:

gem bump --version minor --tag --release

This command will:

• bump the gem version to the next minor (changing the version.rb file)
• commit the change and push it to upstream master
• create a git tag
• push the tag to the upstream remote
• release the new version on RubyGems

Or you can run these commands individually:

gem bump --version minor 
gem tag
gem release