Read post install message from the gemspec and include in the pull request notes

[jordanbyron]

Sometimes maintainers have fun with post install messages (I'm looking at you httparty), but some maintainers use it to warn about breaking changes: doorkeeper-gem/doorkeeper@16e76e6#diff-e6d808b094e36d00159bd8bd183e8968R31

In those cases it would be helpful (and in the above example critical) for those messages to be seen and not missed.

It should be possible to do this by unzipping the built gem file and reading in the compiled gem specification using YAML. Only updated gems would need to do this.