rename primary key sequence only if it exists

cc/ @sgrif @rafaelfranca @senny

When a table has  no sequence in the  for primary key (for example :uuid), it was not possible to rename it without using sql. This fix it.

Gotcha. `test_renaming_table_doesnt_attempt_to_rename_non_existent_sequences`, and I'd wrap the call to `rename_table` in `assert_nothing_raised` (I realized that the tests will fail if anything is raised, but expressing the intent of that line being the thing that was raising an exception is good)