Allow configuring custom web3 instance

[frangio]

Fixes #35

Removes the assumption that there is a global web3 (as in a Truffle environment) by adding a mechanism to manually configure the web3 instance. Also allows using in Truffle migrations (OpenZeppelin/openzeppelin-contracts#1743).

require('openzeppelin-test-helpers/configure')({ web3: ... });

Also where possible replaces web3.utils by the web3-utils package.

[nventuro]

You need to add the new integration test to the .travis.yaml file for it to actually run.

[frangio]

Sorry, let me add some documentation and a changelog entry.

[frangio]

@nventuro Done.

I also changed the API slightly. It is now called configure and receives an object. Additionally, it no longer returns the helpers, forcing you to get them as another step. This makes it clear that you are first expected to configure, and only afterwards use the library.

require('openzeppelin-test-helpers/configure')({ web3 });
const { expectEvent } = require('openzeppelin-test-helpers');

[nventuro]

This is fantastic @frangio, thanks a lot!