Make sure Unicode files are only downloaded once, not repeatedly, for continuous integration
People from the Unicode Consortium pointed out to me that we shouldn't repeatedly download the Unicode data files for continuous integration. I know that we are fine for compiling updates, e.g. locally.
What I want to help somebody check (and fix if necessary) is whether e.g. travis and similar sites repeatedly download the Unicode files or not, and how this may be affected by switching to HTTPS.
Some travis experts, please comment, thanks!