Error running aws-shell in virtualenv

When installing the latest version of awscli and aws-shell in a virtualenv on Debian 9, running aws-shell fails with an error creating the autocomplete index.

First run, creating autocomplete index...
Traceback (most recent call last):
File "/home/aws/aws/lib/python3.5/site-packages/awsshell/utils.py", line 84, in file_contents
with open(filename, mode) as f:
FileNotFoundError: [Errno 2] No such file or directory: '/home/aws/.aws/shell/cache/completions-1.16.14.json'

The Debian packages for awscli and aws-shell work. Here are the versions for those.

aws-shell/stable 0.1.1-2
awscli/stable 1.11.13-1

Details attached for Debian 9. I have also tried Debian 10 and Fedora 28 with similar results.

[python3-virtualenv-aws-debian-9.txt](https://github.com/awslabs/aws-shell/files/2381245/python3-virtualenv-aws-debian-9.txt)

Reproduce the failure in a Python virtualenv using the Debian or Fedora details and the attached report. Start at awsshell/utils.py, especially file_contents and the autocomplete-index creation path, then verify that the first aws-shell run creates and reads completions-1.16.14.json without an exception.

Written by the indexing model from the issue text.