add riak_pb to apps in src/riak_kv.app.src

The riak_pb app is used in riak_kv but was not listed as an application
dependency in the src/riak_kv.app.src file.

Also add missing apps to riak_kv_test_util code that manages
starting/stopping application dependencies for testing.
Merge pull request #730 from basho/vinoski/app-file-additions

add riak_pb to apps in src/riak_kv.app.src