text: Title: Considering any variable that has at least one upper case letter as env var
  Body: ## Problem
  If we define a var containing an upper case letter, like:
  ```yaml
  vars:
      apiKey: abc123
  ```
  
  and we try to use it later as `${apiKey}` it will raise the error:
  
  ```
  ERROR:scanapi.evaluators.string_evaluator:'apiKey' environment variable not set or badly c
  ```
  
  ## Possible Solution
  
  To check if there is any lower case in the word here:
  
  https://github.com/camilamaia/scanapi/blob/master/scanapi/evaluators/string_evaluator.py#L48
  
  ```python
  if any(letter.islower() for letter in variable_name):
    continue
  ```