update "both" command to use actual mpf call
Currently "mpf both" uses os.fork() to call "mpf mc" and "mpf game". But if you have mpf installed differently where you need to run, for example, "python3 -m mpf both", then it will fail since it calls "mpf mc" instead of "python3 -m mpf mc", etc.

diff --git a/mpf/commands/both.py b/mpf/commands/both.py
index c1921ba27..678a220cd 100644
--- a/mpf/commands/both.py
+++ b/mpf/commands/both.py
@@ -1,11 +1,11 @@
 import os
 import platform
 import subprocess
+from importlib import import_module
 
 
 class Command(object):
     def __init__(self, mpf_path, machine_path, args):
-        del mpf_path
         if platform.system() == 'Windows':
             subprocess.Popen('mpf game {} {}'.format(machine_path,
                                                      ' '.join(args)))
@@ -13,8 +13,8 @@ class Command(object):
 
         else:
             if os.fork():
-                os.system('mpf game {} {}'.format(
-                    machine_path, ' '.join(args)))
+                module = import_module('mpf.commands.game')
+                module.Command(mpf_path, machine_path, args)
             else:
-                os.system('mpf mc {} {}'.format(
-                    machine_path, ' '.join(args)))
+                module = import_module('mpfmc.commands.mc')
+                module.Command(mpf_path, machine_path, args)
['mpf/commands/both.py']