benchmark_version

[amcnicho]

asv assigns a value to the benchmark_version dictionary of tests in the generated results JSON for a given environment. By default, this is the hash of the benchmark code itself, but it can be set to any string value. We should intentionally choose a versioning scheme; ideally such that minor refactors (e.g., changing variable names or the setup/setup_cache/teardown methods) don't cause previous valid results to be ignored unless the hashes are overwritten.