Update requirements.txt for missing dependency [inquirer]

[shannonlal]

I have tested this on a Mac and it gives me an error if you do not have inquirer installed. I assume this was a missing dependency.

I followed through the instructions on the home page and got an error

with No Module name 'inquirer'

Here is the output

 agent % python main.py
Traceback (most recent call last):
  File "/Users/xxxxx/agent/main.py", line 2, in <module>
    from generators.topic_generator import TopicGenerator
  File "/Users/xxxxx/agent/generators/topic_generator.py", line 4, in <module>
    from brands import Brand
  File "/Users/xxxx/agent/brands.py", line 2, in <module>
    import inquirer
ModuleNotFoundError: No module named 'inquirer'

I did the following command to fix the issue

pip install inquirer

This PR recommends to add this into the requirements.txt