Avoid assuming the primary key column is called "id"
added pr 117 reference. Thanks for the PR!
pluralizing migration class name

`rails g closure_tree:migration tag`

yields a migration file that contains non pluralized class name:

``` ruby
class CreateTagHierarchy
```

which should be:

``` ruby
class CreateTagHierarchies
```

this allows `rake db:migrate` to run correctly
v5.1.1
fix index name clash for multiple models