Addressed unchecked return value in AllowanceCrowdsale #945 using SafeERC20

[dougiebuckets]

Fixes #945

🚀 Description

Addressed unchecked return value in AllowanceCrowdsale #945 using SafeERC20.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[come-maiz]

Thank you @dougiebuckets. Can you please resolve the conflicts?
And Crowdsale.sol is also using transfer instead of safeTransfer. I think it would make sense to update it in this PR.

[dougiebuckets]

@ElOpio - Thanks for the heads up! Should be all set.

[come-maiz]

This looks good to me.
The reverts of safeTransferFrom are already tested, so I don't think we have to test the revert here again. But certainly that would be bonus points.
Leaving my +1, waiting for the opinion of the second reviewer.