Create update-packages.R

add two new functions to check for old packages and update the miniCRAN versions:
* `old.packages.miniCRAN()` follows code/syntax of `utils::old.packages()`
* `update.packages.miniCRAN()` follows code/syntax of `utils::update.packages()`
* the update function calls `add.packages.miniCRAN()` in `add-packages.R`