The master branch of the repo currently contains the work for the upcoming 3.0 release. For this release, we are removing the current role contracts, which includes MinterRole. Since we are removing this role, we are also removing the ERC20Mintable contract which used it.
There will be an alternative to ERC20Mintable and roles in 3.0, but we haven’t built it yet.
In the meantime, please continue using ERC20Mintable from the 2.5 release of Contracts, which can be found in the repository in the link that @abcoathup provided above.