Fitting with log_gpu_memory=True fails in python3.6.
Bug
Fitting with log_gpu_memory=True in the Trainer fails in python3.6 version.
To Reproduce

Use python3.6 version
Create any trainer with log_gpu_memory=True option.
Then fit it.
See error:

/a/pytorch-lightning/pytorch_lightning/core/memory.py in get_gpu_memory_map()
    237         encoding='utf-8',
    238         capture_output=True,
--> 239         check=True)
    240     # Convert lines into a dictionary
    241     gpu_memory = [int(x) for x in result.stdout.strip().split(os.linesep)]

/usr/lib/python3.6/subprocess.py in run(input, timeout, check, *popenargs, **kwargs)
    421         kwargs['stdin'] = PIPE
    422 
--> 423     with Popen(*popenargs, **kwargs) as process:
    424         try:
    425             stdout, stderr = process.communicate(input, timeout=timeout)

TypeError: __init__() got an unexpected keyword argument 'capture_output'


Code sample
trainer = Trainer(
    log_gpu_memory=True,
   # ....
)
trainer.fit()

Expected behavior
For the same code there is no errors for python3.7
Environment
pytorch:          1.2.0
Ubuntu 18.04
pytorch-lightning:
- installed to pip environment
- commit 7a1df80
- python setup.py develop
- version 0.6.0
python:         3.6.8
cuda: 10.0, V10.0.130
cudnn:          7.6.2
GPU: RTX 2080 TI
Additional context
In the setup.py
python_requires='>=3.6',
But  capture_output is used in  subprocess.run calling, which is valid only for python3.7
See also workaround to maintain python3.6:
https://stackoverflow.com/questions/53209127/