Hotfix for Pipfile case-sensitivity detection

Cherry-pick Pipfile detection fix for 2018.2.1 (#1002)

* Detect pipenv by looking for Pipfile, not pipfile (#994)

On platforms with case-sensitive filesystems, like Linux, these are not
equivalent. pipenv documents that the file should be called Pipfile[0]
and `Pipfile.find()` only finds files matching this exact case[1].

As a result, even if `pipenv --venv` in `cwd` would return success, it
will never be run on Linux, and Code never detects the pipenv. (You can
work around this with `touch pipfile`.) With this change, it's detected
successfully. I believe there's no need to add a backwards-compatibility
check for the old case, because on platforms where the old, incorrect
check worked, so will the new, correct one.

[0] https://docs.pipenv.org/basics/#example-pipfile-pipfile-lock
[1] https://github.com/pypa/pipfile/blob/5acb9ac7/pipfile/api.py#L76-L85

* Prep for 2018.2.1