Fix loading eredis and poolboy when compiled with rebar3

[aboroska]

eredis has to be loaded before eredis_cluster starts. poolboy has to be started as well.

mix.exs lists eredis and poolboy among applications. The rebar3 equivalent is adding eredis and poolboy to applications in eredis_cluster.app.src.

[aboroska]

I missed poolboy in the first version.

[elmirk]

or problem could be solved if put eredis&poolbow in top app appliaitons list, +1 to PR.