Remove explicit dependency on riak_pb

riak_search depends on riak_api, which has a rebar dependency on
riak_pb, so this one is not needed. Moreover, it's conflicting with the
one in riak_api, causing compilation problems in riak.
Remove redundant riak_api dependency

riak_api is already a dependency of riak_kv, which is already listed in
rebar.config
Merge pull request #143 from basho/eas-remove-riak-pb-dep

Remove explicit dependency on riak_pb