Some refactoring to fix Linux support

[valericus]

Hi!

I've tried to use this package with Linux Mint and faced with following exception.

Traceback (most recent call last):
  File "/home/valera/code/personal/project/main.py", line 1, in <module>
    from pygeckodriver import geckodriver_path
  File "/home/valera/code/personal/project/venv/lib/python3.8/site-packages/pygeckodriver/__init__.py", line 37, in <module>
    geckodriver_path = _get_filename()
  File "/home/valera/code/personal/project/venv/lib/python3.8/site-packages/pygeckodriver/__init__.py", line 29, in _get_filename
    path += file_extension
TypeError: can only concatenate str (not "NoneType") to str

So I've looked through source code and found a problem with file_extension variable. Minimal fix to make it work looks like

---      file_extension = None
+++      file_extension = ''

But I think some refactoring makes this code a bit more readable and error-proof.

Fixes #5

[valericus]

Also I've updated update.py script to ignore .asc files.

[spacepost]

I just had to make this same change and was coming out here to write up the same issue. Can confirm that these changes fix the issue I was encountering in linux as well.

[spacepost]

I created an issue for this, but I can't link your PR to it since I didn't create it. You should associate this to the issue: Linux Support Issue #5

[valericus]

I created an issue for this, but I can't link your PR to it since I didn't create it. You should associate this to the issue: Linux Support Issue #5

Done, thank you for the issue.