Remove duplicate regexp phrase: ^\A

Addresses @mastahyeti's comment in #3077: https://github.com/jekyll/jekyll/pull/3077#discussion_r20077150

No clue how the tests passed with that invalid regexp. It's frustrating not being able to CI this on Windows.
Because we strip the drive from the `clean_path`, yes, we have to be drive-agnostic when comparing. Maybe we compare the full `clean_path` and `base_directory` with drive names, then, if they don't overlap the way we prescribe, then we strip the drive name from `clean_path` and stuff it onto `base_directory`?

```ruby
def sanitized_path(base_directory, questionable_path)
  clean_path = File.expand_path(questionable_path, '/')
  unless clean_path.start_with?(base_directory)
    File.join(base_directory, clean_path.sub(/\A\w:\//, '/'))
  else
    clean_path
  end
end
```