Update x/net and x/crypto to latest version in Obfs4proxy and Snowflake

Somebody sent me an issue about this:

https://github.com/tladesignz/IPtProxy/issues/45

For security purposes, golang.org/x/crypto and golang.org/x/net should be updated to latest versions. This also requires the obfs4 and snowflake submodules to do the same thing, so there needs to be some coordination between all three projects for this to happen.

Please let me know, if this is valid, and if there's any problems with that request! Thank you!