For master, don't use hashes to check whether scripts are out-of-date.

If there isn't a hash in version.txt, a script will only be downloaded if it doesn't exist locally. No specific hash checking is done.

To me, this seems like a better solution for users working with master. It relieves us from having to maintain version.txt in master and update it with every commit that affects one or more scripts, and anyone using master likely already has the git repository and can just `git pull` to get the latest versions.

I believe this will take care of https://github.com/ropensci/ecoretriever/issues/23 for anyone using the master version of the Retriever.