use short index name for mysql adapter

During installation of  `copywriting` with rails 4, mysql and refinery from master i am getting this error on migration step:
```bash
rake db:migrate
==  CreateCopywritingTranslationTable: migrating ==============================
-- rename_column("copywriting_phrase_translations", :copywriting_phrase_id, :refinery_copywriting_phrase_id)
rake aborted!
An error has occurred, all later migrations canceled:

Index name 'index_copywriting_phrase_translations_on_refinery_copywriting_phrase_id' on table 'copywriting_phrase_translations' is too long; the limit is 64 characters
```
So this pull request should fix this.