Set IMPL_VERSION from ADAPTER_VERSION environment variable.

This environment variable will be set by the installer script.
I guess you and @cakkineni have already worked this out, but it still seems kinda backwards to me. That means that the adapter won't know it's version unless it's being run within a container and that container has been started by our installer.

Seems like we should keep the version string close to the code it's versioning and then use that information to drive the image tag that is created in Docker.