Add schema_plus to required gems

W agreed to use [postgres](https://github.com/monterail/guidelines/blob/master/toolbox.md), we know that we need indexes #100.

So maybe we should add [schema_plus](https://github.com/lomba/schema_plus) by default?

I'm using it in one project, and I have no problems. It just works.

It automatically creates indexes for every foreign key and allows to create composite keys. Doc is full of examples how to use it, so it shouldn't bring any problems.