Fix a logic error in the the check for appspec.yml when using tgz archives

[bennettp123]

Hi there!

Thanks for merging my previous PR (#73)! Unfortunately, I introduced a logic error in the check for appspec.yml

Currently:

• check for files that do not match appspec.yml
• raise an error if any are found

Unfortunately, there's typically going to be logs of files that aren't named appspec.yml!

This PR fixes the error:

• check for the existence of appspec.yaml, then negate the result

(Thanks @vikasperi for providing the fix!)