Publish latest version to NPM

[jasny]

The version on NPM is two years old. It still uses solidity 0.6.0 instead of 0.8.0 which is supported in the latest version on the master branch.

[jasny]

For anyone wanting to update to the latest version on the master branch, run

npm i -D 'gnosis/mock-contract#master' --save

[akshay-ap]

Any plans to release a version rather than using this as a workaround?