Fix random failures of tests on Travis

- Tests on Travis are randomly failing because schema_migrations table
  does not exist in teardown block.
- Also checked that all other places where we have used
  `ActiveRecord::SchemaMigration.delete_all` we have rescued it, so used
  it here also. This failure was not specifically related to the test
  added in this PR but to overall compatibility migration tests, so
  adding as separate commit.

activerecord/test/cases/migration/compatibility_test.rb

@@ -21,7 +21,7 @@ module ActiveRecord
       teardown do
         connection.drop_table :testings rescue nil
         ActiveRecord::Migration.verbose = @verbose_was
-        ActiveRecord::SchemaMigration.delete_all
+        ActiveRecord::SchemaMigration.delete_all rescue nil
       end
 
       def test_migration_doesnt_remove_named_index