v1.1.0: Exclude mix.exs from mix-test hook

Fix #1 (closed). Exclude mix.exs file from mix-test to prevent an obscure compilation error.

Tested with this commit from jvenom/ex-bexar-county-jail-activity - the mix test hook was skipped as expected.

> git -c user.useConfigOnly=true commit --quiet --allow-empty-message --file -
> git ls-files --stage -- /Users/josh/codebase/elixir/ex-bexar-county-jail-activity/.pre-commit-config.yaml
> git cat-file -s 6a7e476127d362bd1773cf666041306bd83c09ec
> git show --textconv :.pre-commit-config.yaml
[WARNING] The 'rev' field of repo 'https://gitlab.com/jvenom/elixir-pre-commit-hooks' appears to be a mutable reference (moving tag / branch).  Mutable references are never updated after first install and are not supported.  See https://pre-commit.com/#using-the-latest-version-for-a-repository for more details.  Hint: `pre-commit autoupdate` often fixes this.
fix end of files.........................................................Passed
check yaml...........................................(no files to check)Skipped
mixed line ending........................................................Passed
mix format...............................................................Passed
mix test.............................................(no files to check)Skipped