Update manifest.json

[chiefcomm]

Added version key to be compliant with Home Assistant - addresses issue #56

[mk-maddin]

please implement as: ERROR (MainThread) [homeassistant.loader] No 'version' key in the manifest file for custom integration 'variable'. As of Home Assistant 2021.6, this integration will no longer be loaded. Please report this to the maintainer of 'variable'

[Mariusthvdb]

you should simply add that to your local manifest, as this Repo is no longer maintained, and the owner has long since got quiet.

[chiefcomm]

after persisting with this addon I've decided to switch to another https://github.com/snarky-snark/home-assistant-variables does everything and more - I suggest switching.

[Mariusthvdb]

just as a FYI, there is a current fork which is active and maintained: https://github.com/Wibias/hass-variables

and, the 2 are not identical, hass-variables allows for the history attributes, which isn't supported on the snarky integration.