Add require "reason string" for paused contracts

[jaycenhorton]

Fixes #
n/a

🚀 Description

Adds "reason strings" for require statements in pausable contracts for consumption in latest version of solidity/truffle v 0.5

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

[nventuro]

Hey @jaycenhorton, thank you for contributing to OpenZeppelin!

Sadly, we're not doing adding require messages until #888 is resolved, which in turn requires #917.