Update gem version to latest

[ianfixes]

Added

• Publish docker image to GitHub packages on latest branch and after tagging
• Instructions for testing locally with Docker
• Adopt new arduino_ci version that enables CUSTOM_INIT_SCRIPT and USE_SUBDIR environment variables
• Python dependencies for espXX board compilation (unfortunately assumed to be present in image, not installed by board manager)
• Instructions for new environment variable SKIP_LIBRARY_PROPERTIES

Issues fixed

• Fixes Enable CI testing to run against a subdirectory of the repository #3 - test a subdirectory
• Fixes Enable installation of custom library versions prior to testing #5 - allow custom library version installation
• Fixes Improve reference to unit testing documentation #8 - better docs references
• Fixes Default arduino_ci configuration may test platforms/architectures that, according to library.properties, are unsupported #11 - don't test incompatible architectures
• Fixes Improve documentation about default platforms tested and how to override #12 - improve docs
• Fixes Ensure python / python3 / pip exists in the docker image #13 - add python to image