bring store_riak back to life

[Volodymyr Kyrylov]

subj

[Максим Сохацький]

it is alive, just uncomment

    {riak_kv, ".*", {git, "git://github.com/spawnproc/riak_kv.git", {tag,"1.3.2-voxoz"}}},

[Volodymyr Kyrylov]

i think we need to make it pluggable using an additional dependency like kvs_riak so the users won't need to touch external repos

[Максим Сохацький]

riak and kai backends tested and they are ok.