Re-enable solidity coverage

- Upgrade version
- Re-enable in travis.yml
- Move mocks to contracts folder for instrumentation

test/SafeERC20.test.js

@@ -4,7 +4,7 @@ require('chai')
   .use(require('chai-as-promised'))
   .should();
 
-const SafeERC20Helper = artifacts.require('./mocks/SafeERC20Helper.sol');
+const SafeERC20Helper = artifacts.require('mocks/SafeERC20Helper.sol');
 
 contract('SafeERC20', function () {
   beforeEach(async function () {