write .cache/keybase/service.version on service startup
write .cache/keybase/service.version on service startup

This provides an easy way for updated clients to find out what version
of the service is currently running, so that they can bounce it if need
be.

Closes https://github.com/keybase/client/issues/415.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client