Update rebar.config to restrict erlang version to R16
Update rebar.config to restrict erlang version to R16

The latest development has been against R16 and the HEAD of develop no
longer builds with versions less than R16.

basho/riak,yangchengjian/riak,glickbot/riak,GabrielNicolasAvellaneda/riak,DavidAlphaFox/riak,amyvmiwei/riak,authbox-lib/riak,elkingtonmcb/riak,biddyweb/riak,laurenrother/riak