add index for address_current_token_balances

apps/explorer/priv/repo/migrations/20190208143201_add_index_on_address_hash_block_number_and_token_contract_address_hash_for_current_token_balance.exs

@@ -0,0 +1,7 @@
+defmodule Explorer.Repo.Migrations.AddIndexOnAddressHashBlockNumberAndTokenContractAddressHashForCurrentTokenBalance do
+  use Ecto.Migration
+
+  def change do
+    create(index(:address_current_token_balances, [:address_hash, :block_number, :token_contract_address_hash]))
+  end
+end