Fix DB error when trying to tag a repository

Upon tagging a repository, an error due to a missing migration occurred:

```
    PG::Error: ERROR:  column "taggings_count" does not exist
        LINE 1: UPDATE "tags" SET "taggings_count" = COALESCE("taggings_coun...
```

Ran `rake acts_as_taggable_on_engine:install:migrations`, removed the first
migration that created the `tags` and `taggings` table, and added the rest.

Closes #1512
Closes https://github.com/gitlabhq/gitlabhq/issues/6867
Closes https://github.com/gitlabhq/gitlabhq/issues/9194