Related to this blog post, @radix commented:

Python's pip package manager also doesn't retry, and I don't really understand why not. (this is one of the most common reasons for a travis job to fail for me, on my Python projects). It seems to me some conservative number of retries should definitely be done, say, 3.

Adding retry logic on download shouldn't be difficult, and ideally would be done with some kind of a config option to control the number of retries (and possibly the delay between them).