changed constructed/allowed tx 'version' from '1.0' to '2.0' (#2048)

- Created three new JSON Schema files for the v2.0 transaction schema, based on the v1.0 files. The only change is the allowed value of version (from "^1\\.0$" to "^2\\.0$").
- I didn't delete the v1.0 JSON Schema files because we might want those some day (to validate old transactions).
- Updated the __init__.py in the directory with the JSON Schema files so that it now uses the version 2.0 JSON Schema files.
- Updated all relevant tests. I only found one, in tests/validation/test_transaction_structure.py
- Updated VERSION in common/transaction.py
- Checked to make sure the example HTTP API docs show "version" with value "2.0". They do.
- Updated the docs page about "The Transaction Model". It just points to the IPDB Transaction Spec.
- If someone submits a transaction with "version" having value "1.0" then the error message comes from the JSON Schema checker.

docs/server/source/data-models/transaction-model.rst

@@ -2,21 +2,3 @@ The Transaction Model
 =====================
 
 See the `IPDB Transaction Spec <https://github.com/ipdb/ipdb-tx-spec>`_.
-
-
-The Transaction Schema
-----------------------
-
-BigchainDB checks all transactions (JSON documents)
-against a formal schema defined
-in some `JSON Schema <http://json-schema.org/>`_ files.
-Those files are part of the 
-`IPDB Transaction Spec <https://github.com/ipdb/ipdb-tx-spec>`_.
-Their official source is the ``tx_schema/`` directory
-in the `ipdb/ipdb-tx-spec repository on GitHub
-<https://github.com/ipdb/ipdb-tx-spec>`_,
-but BigchainDB Server uses copies of those files;
-those copies can be found
-in the ``bigchaindb/common/schema/`` directory
-in the `bigchaindb/bigchaindb repository on GitHub
-<https://github.com/bigchaindb/bigchaindb>`_.