update release.sh and release.rst to include pantsbuild.pants.testinfra

Moving on, we'll publish two packages at the same time: pantsbuild.pants
and pantsbuild.pants.testinfra to PyPi. While they're sim-released, users
of pants can choose to install pants only if they don't have any
customization to pants thus avoiding the need of writing additional test
cases to which the pantsbuild.pants.testinfra would be helpful.

Testing Done:
I'm not the owner of pantsbuild.pants package on PyPi, so all my testing are local:

build-support/bin/release.sh -t
build-support/bin/release.sh -n

tested both. For the '-n' dry-run, I've also manually injected some failures at pants, pants_testinfra build stages, as well some failures in their testing commands, to have verified the proper command chaining behavior of the bash script.

Bugs closed: 539

Reviewed at https://rbcommons.com/s/twitter/r/1004/