Set IMPL_VERSION from ADAPTER_VERSION environment variable.

This environment variable will be set by the installer script.
So this will be set by the remote installer [via Chaitanya magic](https://github.com/CenturyLinkLabs/panamax-remote-agent-installer/pull/8/files) using the docker image version tag for the image. Community adapters might take a different approach, but this seems like a really easy way for our releases. No 'version bump' commit messages or accidentally forgetting to bump the version when we do a release.