Location of ABI needs to be variablized

[cliffhall]

The output location for ABIs can be changed from the default in Truffle.js:

module.exports = {
    contracts_build_directory: path.join(__dirname, "src/abi"),
}

This often has to be done when building a React application using Drizzle, since React only has access to files under its src folder at compile time. Consequently, this leads to a problem with unit tests that use the truffle-events library, which hardcodes the path to the contract ABI:

  getAbi: function(contractName) {
    let c = JSON.parse(fs.readFileSync(`./build/contracts/${contractName}.json`, `utf8`));
    return c.abi;
  }

Resulting in the following error:

  Error: ENOENT: no such file or directory, open './build/contracts/MyContract.json'
      at Object.openSync (fs.js:448:3)
      at Object.readFileSync (fs.js:348:35)
      at Object.getAbi (node_modules/truffle-events/index.js:6:27)