ensured dependencies are only re-downloaded if outdated

thus reducing the time it takes to set up a dev instance

via
http://www.oddmuse.org/cgi-bin/oddmuse/Download_Using_Curl_And_Caching

note, however, that neither `wrap_jslib` nor the Python-based retrieval
(via `ibuilder`) currently take advantage of caching
Merge pull request #1067 from FND/depcache

ensured dependencies are only re-downloaded if outdated