Add a versioning scheme for the environment/dependencies

There's a recurring problem of dependencies being added and then someone getting the latest code and not knowing this is the case. A simple control mechanism would be to have ./run_tests.sh keep track of an environment version that was incremented every time a new dependency was introduced and force a rebuild if it found the current environment version (tracked in a hidden file) either out-of-date or missing. It could additionally force a rebuild on changes of any major version revision (e.g. a new release). A yes/no option could be provided to allow the developer to skip the version update if they so desire.