add missing apps to src/riak_kv.app.src

The basho_stats, eleveldb, pbkdf2 and poolboy apps are used in riak_core
but were not listed as application dependencies in the
src/riak_core.app.src file.
Merge pull request #453 from basho/vinoski/app-file-additions

add missing apps to src/riak_kv.app.src