Make version_code into a SharedValue

[bensig]

Would be very helpful for fastfile integration if this were made into a shared value to use in other actions:

module SharedValues VERSION_CODE ||= :VERSION_CODE

then something like

` # Store the new number in the shared hash
version_code = params[:version_code].to_s.strip
version_code = Actions.sh("#{command_prefix} agvtool what-version", log: false).split("\n").last.strip if version_code.to_s == ""

    return Actions.lane_context[SharedValues::VERSION_CODE] = build_number