Update version.go

[c-f]

Reason

This PR changes the ProtocolVersion to a variable to allow dynamic changes if chisel is used as a library.

Currently chisel is used as a library to configure server and client dynamically. Especially if additional features were added the protocolversion cannot be changed to indicate the existance.

Additionally Symantec flags traffic from/to chisel as suspicious if chisel-v3 is matched in the traffic.

Thanks, :)

[c-f]

Hi, any updates or comments on my request ?

Thanks, :)

[jpillora]

Thanks!