Fix versioning to be accepted into the Rack community repository

[AndrewBelt]

Rack v0.5.1 will include a plugin manager, and plugins need to follow the versioning guidelines in order to be accepted. You can use this example plugin Makefile and Tutorial.cpp file as a reference for making sure the version is the same in the C++, Makefile, and JSON manifest.

• Your plugin has no version defined in a C++ file. You must set plugin->version using the Tutorial.cpp file as a reference.

Once you've fixed this, send a PR to the Rack community repository, fixing the version is applicable and removing the "disabled" property in the JSON manifest file if present.

[stellare-modular]

Fixed with version 0.5.1