Subprocess is not cleaned up

Hello! Cool project, thank you.

After running the tests
```
python3.7 socrates.py -p 1 /philosophers
```

Grep'ing processes with the name `philo`
```
ps aux | head -n +1
ps aux | grep "philo"
```
Outputs:
```
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
user42 7265 0.4 0.0 47680 856 pts/1 Sl 20:32 0:00 /philosophers/philo_one/philo_one 4 311 150 150
user42 7273 0.4 0.0 47680 876 pts/1 Sl 20:32 0:00 /philosophers/philo_one/philo_one 4 311 150 150
user42 7280 0.4 0.0 47680 888 pts/1 Sl 20:32 0:00 /philosophers/philo_one/philo_one 4 311 150 150
user42 7288 0.4 0.0 55876 948 pts/1 Sl 20:33 0:00 /philosophers/philo_one/philo_one 5 600 150 150
user42 7297 0.4 0.0 55876 876 pts/1 Sl 20:33 0:00 /philosophers/philo_one/philo_one 5 600 150 150
user42 7305 0.4 0.0 55876 836 pts/1 Sl 20:33 0:00 /philosophers/philo_one/philo_one 5 600 150 150
user42 7386 0.0 0.0 16952 1060 pts/1 S+ 20:33 0:00 grep --color=auto --exclude-dir=.bzr --exclude-dir=CVS --exclude-dir=.git --exclude-dir=.hg --exclude-dir=.svn --exclude-dir=.idea --exclude-dir=.tox philo_one
```

I think this is happening because the program is running in a shell and process.kill() seems to kill only the shell.

A posible fix is to [import shlex](https://docs.python.org/3/library/shlex.html) and in `assert_runs_for_at_least()` set shell to False
```
process = subprocess.Popen(shlex.split(command), stdout=subprocess.DEVNULL, shell=False)
```

This will work for philo_one and philo_two because they use threads.

philo_three it's creating multiple subprocesses, so they need to be cleaned up before returning.

Tested on `Xubuntu 18.04` with `python3.7`

Edit:

Just saw the function `processes_still_running()` which should clean the processes and I don't know why it's not working on my machine, will test it more since I'm currently working on philosophers project.

Start with assert_runs_for_at_least() in socrates.py and inspect how processes_still_running() handles the subprocesses launched by the philosophers tests. Reproduce the command shown with Python 3.7, including philo_three, and verify that no philosopher processes remain after the tests finish.

Written by the indexing model from the issue text.