Upgrade snowflake bridge to 0.3.5.15 by 2021-06-15 for TROVE-2021-003, -004, -005, -006

https://lists.torproject.org/pipermail/tor-talk/2021-June/045737.html

In around two weeks–likely on the 14th or 15th­– we plan to put out new stable Tor releases to fix issues in all currently released versions of Tor. There are three issues that will be fixed, with severity levels between "Medium" and "High" according to our classification system. The most severe issue, by our reckoning, is a denial-of-service issue affecting onion service clients. We'll share more details after people have time to patch. To the best of our knowledge, these vulnerabilities are not being exploited in the wild.

The new releases will be 0.3.5.15, 0.4.4.9, 0.4.5.9, 0.4.6.5. The issues to be fixed are TROVE-2021-003 through TROVE-2021-006. When these releases are out, we will recommend that everybody upgrade, including clients and relays.