I just merged a [PR](https://github.com/ledell/cvAUC/pull/4) to move `data.table` to Imports, but let's also move `ROCR` to Imports. References: Reasoning [here](https://stackoverflow.com/questions/8637993/better-explanation-of-when-to-use-imports-depends#8638902) and explanation [here](https://stackoverflow.com/a/65605275)
