updated version history for 3.20
updated version history for 3.20

utiasASRL/easyloggingpp,arvidsson/easyloggingpp,chenmusun/easyloggingpp,spqr33/easyloggingpp,utiasASRL/easyloggingpp,simonhang/easyloggingpp,hellowshinobu/easyloggingpp,simonhang/easyloggingpp,lisong521/easyloggingpp,spthaolt/easyloggingpp,arvidsson/easyloggingpp,chenmusun/easyloggingpp,dreal-deps/easyloggingpp,hellowshinobu/easyloggingpp,lisong521/easyloggingpp,dreal-deps/easyloggingpp,spqr33/easyloggingpp,spthaolt/easyloggingpp,chenmusun/easyloggingpp,hellowshinobu/easyloggingpp,blankme/easyloggingpp,utiasASRL/easyloggingpp,blankme/easyloggingpp,dreal-deps/easyloggingpp,spqr33/easyloggingpp,lisong521/easyloggingpp,spthaolt/easyloggingpp,simonhang/easyloggingpp,arvidsson/easyloggingpp,blankme/easyloggingpp