Rename PingSender to HeartbeatRequester

Signed-off-by: Trevor Ackerman <tackerman@pivotal.io>
Metron sends HeartbeatRequests to Dropsonde

[#81724838]

Signed-off-by: Trevor Ackerman <tackerman@pivotal.io>
Bump dropsonde

[#81724838]

Signed-off-by: John Tuley <jtuley@pivotal.io>