Update README to refer to a prior version of joinmarket

[sanket1729]

The current build fails with the following error: I don't think this project is actively maintained so I just added a checkout to where I know it works.

  File "rangeproof.py", line 291, in <module>
    run_test_rangeproof(value, rangebits)
  File "rangeproof.py", line 257, in run_test_rangeproof
    rp.generate_proof(proofval)
  File "rangeproof.py", line 57, in generate_proof
    pc = PC(encode(self.value, 256, minlen=32), blinding=self.gamma)
  File "/home/sanket1729/bulletproofs-poc/vectorpedersen.py", line 18, in __init__
    self.h = getNUMS(255).serialize() if not h else h
AttributeError: PublicKey instance has no attribute 'serialize'

[AdamISZ]

Indeed, this is not maintained; I listened to someone's advice that it'd be better not to have 'test' code out that people might incorrectly treat as actually usable, hence I removed the backend bitcoin functions and just left it.

Thanks for the attention though; I guess that, yes, this PR would be a way to try to run it, if you want to do so to help you figure out some details of what's going on in the bulletproofs range proof setup. No guarantees that this is actually functionally correct, bear that in mind.