Add record to track rubygem total downloads
Add record to track rubygem total downloads

80 rubygems seem to missing following record used to track total
downloads of a rubygem:
```
GemDownload.where(rubygem_id: rubygem.id, version_id: 0)
```
All of these gems have created at date before 2012, while we started
adding above record in 2016. We may have missed adding rubygem record
in backfill because they didn't to have any versions at the time.

We need to create rubygems record for unindexed rubygems as well,
since someone can start using these names in future. for example:
rexml was reserved name but now it is being used.

sonalkr132/rubygems.org,sonalkr132/rubygems.org,sonalkr132/rubygems.org,rubygems/rubygems.org,sonalkr132/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org