Change to Subversion PMC rule for approving backports

[Julian Foad]

Recently there have not been enough developers willing and able to test 
and approve proposed fixes for back-port to the supported release branches.

We have just been discussing this on #svn-dev [1]. Rather than delay 
forever, myself, stsp and brane decided that in line with "silence 
implies consent", we can go ahead with merging the proposed backports 
even if they have fewer than 3 votes.

We will go ahead now.


[1] discussion around 
https://colabti.org/irclogger/irclogger_log/svn-dev?date=2019-07-18#l80

- Julian