Feature/performance update

[techalchemy]

• Cuts runtime of normal commands by about 2/3

Before

 ~/g/pipenv  master  time -p pipenv --venv
(1339ms)  Sat 03 Nov 2018 08:04:36 PM EDT
/home/hawk/.virtualenvs/pipenv-MfOPs1lW
real 1.31
user 1.20
sys 0.09

After

 ~/g/pipenv  feature/performance-update  time -p pipenv --venv
(543ms)  Sat 03 Nov 2018 08:04:08 PM EDT
/home/hawk/.virtualenvs/pipenv-MfOPs1lW
real 0.50
user 0.46
sys 0.03

Signed-off-by: Dan Ryan dan@danryan.co