Builder methods for post conditions not exported in index.js

[friedger]

Only the three transaction methods are exported in index.js

export { makeSTXTokenTransfer, makeSmartContractDeploy, makeContractCall } from './builders';

[yknl]

Addressed in https://github.com/blockstack/stacks-transactions-js/commit/6125026d256d44da0aff966ef82189ca30c1e7b1