Cleanups to recent patch to pants goal depmap

Incorporated feedback from Patrick after submission of https://rbcommons.com/s/twitter/r/661/

Remove is_delegate attribute from AbstractTarget
Remove stray debug print message in a test

Testing Done:
ci passed at https://travis-ci.org/pantsbuild/pants/builds/30079790

Also ran pants goal depmap by hand on a dependencies() target in our repo

Bugs closed: 359

Reviewed at https://rbcommons.com/s/twitter/r/692/
Update the release docs and unify the pants semver.

This centralizes the pants version editing to
src/python/pants/version.py and firms up a manual release
process that will be dogfooded after this commit.

Outstanding work includes automation of release testing
and a scheme for editing & publishing a CHANGELOG.

Testing Done:
Ran the release test command block and that works - tail lines included in the docs itself.
Also burned a test site, see: http://pantsbuild.github.io/jsirois/release.html

Bugs closed: 357

Reviewed at https://rbcommons.com/s/twitter/r/690/
doc: committing someone else's change: not always straightforward

'./rbt patch -c 123' doesn't always work, so don't suggest
it so prominently

Testing Done:
Trying out the steps on this very commit.

Reviewed at https://rbcommons.com/s/twitter/r/699