The get_key_value method is currently only avaiable in nightly rust.
As only this feature is required it worth to refactor and enable building
in the stable channel.
The method was removed by first getting the value from the CGROUPS hashmap,
then key is get by iterating over all the keys. The checks for an empty key and
key == "devices" were moved out of the hashmap block.
The README.md was updated as well to detail the instructions for stable rust.
Signed-off-by: Erich Cordoba <erich.cordoba.malibran@intel.com>
