message: follow_me: small improvement
  We only need the placeholder once, `using namespace` is not really
  needed.
diff: namespace dronecode_sdk {
  -using namespace std::placeholders; // for `_1`
  -
  FollowMeImpl::FollowMeImpl(System &system) : PluginImplBase(system)
  {
  // (Lat, Lon, Alt) => double, (vx, vy, vz) => float
  @@ -22,8 +20,9 @@ FollowMeImpl::~FollowMeImpl()
  void FollowMeImpl::init()
  {
  - _parent->register_mavlink_message_handler(MAVLINK_MSG_ID_HEARTBEAT,
  - std::bind(&FollowMeImpl::process_heartbeat, this, _1),
  + _parent->register_mavlink_message_handler(
  + MAVLINK_MSG_ID_HEARTBEAT,
  + std::bind(&FollowMeImpl::process_heartbeat, this, std::placeholders::_1),
  static_cast<void *>(this));
  }
  