Teach PluginTestEnvironment to play nicely with Bundler

* Add a few, basic sanity checks.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Add missing Logger require

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Fix obsolete usage of Rake::GemPackageTask

Signed-off-by: Alex Coles <alex@alexbcoles.com>