Update core-agent version to 1.5.0

[jrothrock]

Updates the core agent version to use 1.5.0.

Changes to the core-agent include flushing batched payloads on receiving of termination signals, capturing latency and allocation metrics for background jobs, as well as a few changes to the exit codes of the core agent.

This core agent docs will also be updated and can be found here:
https://scoutapm.com/docs/core-agent

With updates to the core-agent, now flushing payloads on a couple signals, we need to wait for the process to terminate in the tests.