Backout the update to 3.31 and "update" to 2.29.

This is the last version that doesn't use to_string(). to_string() is a
c++11 function and gcc 4.8 on 9.3-RELEASE, used by compiler:c++11-lib,
doesn't provide this function.

Add small blob mentioning this.
Bump portepoch due to version going back.

Reported by: pkg-fallout@