Update requirements.txt to include requests

[perfectra1n]

Currently as it is, the Docker container built from the Dockerfile exits with the following error:

Traceback (most recent call last):
  File "/usr/local/bin/vmware_exporter", line 5, in <module>
    from vmware_exporter.vmware_exporter import main
  File "/usr/local/lib/python3.7/site-packages/vmware_exporter/vmware_exporter.py", line 21, in <module>
    import requests
ModuleNotFoundError: No module named 'requests'

This PR includes requests in the requirements.txt file.