Prepare a release for pantsbuild.pants 0.0.27.

This pulls in a bugfix for python thrift generation.

Testing Done:
Release dry run was good:
```console
$ ./build-support/bin/release.sh -n
```

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/44511402

Bugs closed: 877

Reviewed at https://rbcommons.com/s/twitter/r/1518/