Stringify IndexDefinition column names

Can't run db:migrate because schema_migrations indexes the `:version` column by passing the name of that column as a symbol. This change ensure that the column_names argument to add_index gets stringified before constructing IndexDefinitions. Since other adapters get column names from querying the database, they don't have to worry about rogue symbols.
````
* Execute db:migrate
rake aborted!
undefined method `+' for :version:Symbol
/Users/riley/.rbenv/versions/1.9.3-p327/gemsets/fokus/gems/activerecord-3.2.12/lib/active_record/connection_adapters/abstract/schema_statements.rb:554:in `block in quoted_columns_for_index'
/Users/riley/.rbenv/versions/1.9.3-p327/gemsets/fokus/gems/activerecord-3.2.12/lib/active_record/connection_adapters/abstract/schema_statements.rb:554:in `map'
/Users/riley/.rbenv/versions/1.9.3-p327/gemsets/fokus/gems/activerecord-3.2.12/lib/active_record/connection_adapters/abstract/schema_statements.rb:554:in `quoted_columns_for_index'
/Users/riley/.rbenv/versions/1.9.3-p327/gemsets/fokus/gems/activerecord-3.2.12/lib/active_record/connection_adapters/abstract/schema_statements.rb:578:in `add_index_options'
/Users/riley/Projects/gems/nulldb/lib/active_record/connection_adapters/nulldb_adapter.rb:169:in `add_index'
````