Bump @openzeppelin/contracts from 2.3.0 to 2.4.0

[dependabot-preview]

Bumps @openzeppelin/contracts from 2.3.0 to 2.4.0.

Changelog

Sourced from @openzeppelin/contracts's changelog.

2.4.0 (2019-10-29)

New features:

• Address.toPayable: added a helper to convert between address types without having to resort to low-level casting. (#1773)
• Facilities to make metatransaction-enabled contracts through the Gas Station Network. (#1844)
• Address.sendValue: added a replacement to Solidity's transfer, removing the fixed gas stipend. (#1962)
• Added replacement for functions that don't forward all gas (which have been deprecated): (#1976)
  • PullPayment.withdrawPaymentsWithGas(address payable payee)
  • Escrow.withdrawWithGas(address payable payee)
• SafeMath: added support for custom error messages to sub, div and mod functions. (#1828)

Improvements:

• Address.isContract: switched from extcodesize to extcodehash for less gas usage. (#1802)
• ERC20 and ERC777 updated to throw custom errors on subtraction overflows. (#1828)

Deprecations:

• Deprecated functions that don't forward all gas: (#1976)
  • PullPayment.withdrawPayments(address payable payee)
  • Escrow.withdraw(address payable payee)

Breaking changes:

• Address now requires a minimum Solidity compiler version of 0.5.5. (#1802)
• SignatureBouncer has been removed from drafts, both to avoid confusions with the GSN and GSNRecipientSignature (previously called GSNBouncerSignature) and because the API was not very clear. (#1879)

Commits

• cdf655f 2.4.0
• f5c95cc add changes to minimum solidity compiler version in the changelog
• c4bb7b7 remove beta notice
• 1eb80dd move changelog entry to new features section
• feaf051 added feature availability info to documentation
• 4c997ea make PullPayment.withdrawPaymentsWithGas external
• 1c220e1 Forward all gas on PullPayment withdrawal (#1976)
• ab4d43c fix script for preparing contracts package
• 875f179 Update access-control.adoc (#1968)
• e84e191 Fixes dead link in Readme. (#1969)
• Additional commits viewable in compare view

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