Add support for CircleCI

[andrebrait]

Context

This adds support for CircleCI for this plugin. It's useful for getting branch name and revision number when running on CircleCI.

Contributor Checklist

• [:x:] Added relevant integration or unit tests to verify the changes
• [:heavy_check_mark:] Update the Readme or any other documentation (including relevant Javadoc)
• [:heavy_check_mark:] Ensured that tests pass locally: mvn clean package
• [:heavy_check_mark:] Ensured that the code meets the current checkstyle coding style definition: mvn clean verify -Pcheckstyle -Dmaven.test.skip=true -B

Notes

I couldn't find any tests that tested that sort of thing, hence why I didn't add any.

[TheSnoozer]

That's a good one! Thanks for the fix!

[TheSnoozer]

Hello,
thank you for your contribution and sorry for the delay (I was on a vacation).

Changes look good for me and thanks for the env.containsKey("CI") change since I would suspect many other CI environments might use this variable...