dependency_script - only verify checksum of new downloads

[peterjc]

This is a refinement to #538 from earlier today, which has been working nicely but makes using planemo dependency_script painfully slow if you already have the files downloaded and cached because it will verify their checksums.

I'm wondering about going further and putting this under a --verify switch (like the commented out one proposed for planemo shed_lint), since as used on my TravisCI setup the checksums are run twice (once via planemo dependency_script and again via bash dep_install.sh).

[peterjc]

Updated the deb_install.sh output to also only run shasum -a 256 ... if the file has been freshly downloaded.

This should solve the potential overhead on my TravisCI usage - the files should be downloaded and the checksum will be verified once only.

[jmchilton]

Awesome - thanks for the refinement. This seems like a good compromise.