text: Title: Non-string value in zappa_settings.environment_variables caused AttributeError
  Body: Dear Zappatistas, 
  
  I came across this minor thing: I had the following in my zappa_settings.json
  `"environment_variables": {"FLASK_DEBUG": 0}`
  I tried `zappa update dev` and got back 
  ```
  ValueError("The following environment variables are not strings: {}".format(", ".join(non_strings)))
  ValueError: The following environment variables are not strings: FLASK_DEBUG
  ``` 
  followed by 
  ```
  print("Error: {}".format(e.message))
  AttributeError: 'ValueError' object has no attribute 'message'
  ```
  
  It worked fine with:
  `"environment_variables": {"FLASK_DEBUG": "0"}`
  
  ## Expected Behavior
  Should properly report error without crashing. 
  
  ## Actual Behavior
  Throws an exception about the exception. 
  
  ## Possible Fix
  in cli.py line 1478
  replace             
  ```raise ValueError("The following environment variables are not strings: {}".format(", ".join(non_strings)))```
  with 
  ```raise ValueError("The following environment variables are not strings: {}".format(", ".join([str(non_string) for non_string in non_strings])))```
  
  Also, it seems like ValueError does not have a message attribute, but can be printed like so str(e)
  
  ## Your Environment
  * Zappa version used: 0.45.1
  * Operating System and Python version: Windows10 Python3.6
  
  