Issue description

The documentacion says that the PIPENV_SPINNER environment variable can modify the animation shown while performing long time operations, but pipenv ignores that value.

Expected result

$ export PIPENV_SPINNER=arrow3
$ pipenv lock
▹▹▹▹▹ Locking... # <-- arrow3 spinner style

Actual result

$ export PIPENV_SPINNER=clock
$ pipenv lock
⠙ Locking... # <-- dots spinner style

Steps to replicate

1. export the PIPENV_SPINNER variable to any valid spinner value.
2. run pipenv
3. observe that the dots (or the bouncingBar) spinner style is still present.