Pipenv crash when the virtualenv is corrupt 
A virtualenv can become corrupt if the version of python used to create the virtualenv is no longer available.

Then upon invoking pipenv, this can happen:

```pytb
  Traceback (most recent call last):
  File "/usr/bin/pipenv", line 11, in <module>
    load_entry_point('pipenv==2018.11.15.dev0', 'console_scripts', 'pipenv')()
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 64, in new_func
    return ctx.invoke(f, obj, *args, **kwargs)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/usr/lib/python3.7/site-packages/pipenv/vendor/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/usr/lib/python3.7/site-packages/pipenv/cli/command.py", line 319, in lock
    ensure_project(three=state.three, python=state.python, pypi_mirror=state.pypi_mirror)
  File "/usr/lib/python3.7/site-packages/pipenv/core.py", line 590, in ensure_project
    crayons.green(shorten_path(path_to_python)),
TypeError: __str__ returned non-string (type NoneType)
```

This is the code in question:

```py

            if project.required_python_version:
                path_to_python = which("python") or which("py")
                if path_to_python and project.required_python_version not in (
                    python_version(path_to_python) or ""
                ):
                    click.echo(
                        "{0}: Your Pipfile requires {1} {2}, "
                        "but you are using {3} ({4}).".format(
                            crayons.red("Warning", bold=True),
                            crayons.normal("python_version", bold=True),
                            crayons.blue(project.required_python_version),
                            crayons.blue(python_version(path_to_python)),
                            crayons.green(shorten_path(path_to_python)),
                        ),
                        err=True,
                    )
```

`python_version(path_to_python)` returns `None`. This causes `crayons.blue(python_version(path_to_python))` to crash. Proposed fix in #3408.
diff --git a/pipenv/core.py b/pipenv/core.py
index 7f04ac49..a30cf8dd 100644
--- a/pipenv/core.py
+++ b/pipenv/core.py
@@ -572,7 +572,7 @@ def ensure_project(
                             crayons.red("Warning", bold=True),
                             crayons.normal("python_version", bold=True),
                             crayons.blue(project.required_python_version),
-                            crayons.blue(python_version(path_to_python)),
+                            crayons.blue(python_version(path_to_python) or "unknown"),
                             crayons.green(shorten_path(path_to_python)),
                         ),
                         err=True,
['pipenv/core.py']