Use Truffle artifacts.require correctly

[frangio]

As the documentation says, the argument to artifacts.require should be the contract name, and not the file name.

I think Truffle supports the file name because it worked this way originally, but we should move to the new way.

This affects all tests. Example:
https://github.com/OpenZeppelin/zeppelin-solidity/blob/cda518d35379bb6136a0a5f314a4b8b6fadae4b9/test/StandardToken.test.js#L4

Should be StandardTokenMock = artifacts.require('StandardTokenMock').