Added missing index for variant_id and currency on spree_prices

Query to get products from a taxon
```
SELECT COUNT(*) FROM "spree_products" INNER JOIN "spree_variants" ON "spree_variants"."product_id" = "spree_products"."id" AND "spree_variants"."is_master" = 't' AND "spree_variants"."deleted_at" IS NULL INNER JOIN "spree_prices" ON "spree_prices"."variant_id" = "spree_variants"."id" WHERE "spree_products"."deleted_at" IS NULL AND ("spree_products".deleted_at IS NULL or "spree_products".deleted_at >= '2014-01-20 15:33:58.341590') AND ("spree_products".available_on <= '2014-01-20 15:33:58.341879') AND "spree_products"."id" IN (SELECT spree_products_taxons.product_id FROM "spree_products_taxons" INNER JOIN "spree_taxons" ON "spree_taxons"."id" = "spree_products_taxons"."taxon_id" WHERE "spree_taxons"."id" IN (8970)) AND (spree_prices.amount IS NOT NULL) AND "spree_prices"."currency" = 'EUR';
``` 

Before:
```
Time: 43.675 ms
```

After adding indexes:

```
Time: 2.752 ms
```