Use the packed bin as a fallback

Youtube-dl must be present in the path. If it is not, the packed binary
is used.
Prefer 'unless' over 'if not'

When returning the execution, a new line character is appended to the
command. Executing 'chop' trim it.
Merge pull request #4 from ssedano/packed-bin-fallback

Prefer installed youtube-dl bin