change to_sym to to_s to avoid invalid sql in postgres

Arturo::Feature.to_feature was generating invalid sql in postgres. For example:

```
Arturo.feature_enabled_for?(:multiuser, user)

 Arturo::Feature Load (35.3ms)  SELECT "features".* FROM "features" WHERE "features"."symbol" = "features"."multiuser" LIMIT 1
ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column features.multiuser does not exist
```
Using a string rather than symbol to look up the feature fixes this. The generated sql now has single quotes rather than double quotes around the symbol name, so postgres no longer thinks it's a column name.