Update release notes for 1.3.1
Update release notes for 1.3.1

Fixes #522

Updates describing bugfixes for v1.3.1.

basho/riak_cs,basho/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,dragonfax/riak_cs,sdebnath/riak_cs,yangchengjian/riak_cs,yangchengjian/riak_cs,GabrielNicolasAvellaneda/riak_cs,sdebnath/riak_cs,sdebnath/riak_cs,sdebnath/riak_cs,basho/riak_cs,basho/riak_cs_lfs,GabrielNicolasAvellaneda/riak_cs,dragonfax/riak_cs,dragonfax/riak_cs,dragonfax/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,laurenrother/riak_cs,yangchengjian/riak_cs,yangchengjian/riak_cs,basho/riak_cs,laurenrother/riak_cs,basho/riak_cs_report