[#52225601] Ignore any existing Bundler env vars

Ignore any existing Bundler environment variables when querying the
currently installed versions of Bundler. This is required when Puppet itself
is run under Bundler - in which case you would get the version of Bundler
that it was originally instantiated with.
Merge pull request #6 from gds-operations/52225601-ignore_bundler_env

[#52225601] Ignore any existing Bundler env vars