Prepare the 0.0.24 release.

Bump version & complete the 0.0.24 CHANGELOG section.

Testing Done:
```console
$ ./build-support/bin/release.sh -n
[== Performing a dry run release - no artifacts will be uploaded. ==]
...
[== Building package pantsbuild.pants-0.0.24 with target '//src/python/pants:pants-packaged' ... ==]
...
[== Building package pantsbuild.pants.testinfra-0.0.24 with target '//src/python/pants:test_infra' ... ==]
...
[== Installing and testing package pantsbuild.pants-0.0.24 ... ==]
...
[== Installing and testing package pantsbuild.pants.testinfra-0.0.24 ... ==]
...
[== Dry run release succeeded. ==]
```

Bugs closed: 597, 606

Reviewed at https://rbcommons.com/s/twitter/r/1069/
Sync of https://github.com/pantsbuild/pants/commit/a9a3ae8c645ee2c97c0073203f263bccc90de4c9