[Github Pod Metrics] Add language

I decided to use the `language` field in the response of `/repos/{name}/{repo}`. Both because it means not making an extra call to `/repos/{name}/{repo}/languages` and because I trust Github can figure out the majority language of a repo better than me. 

Closes: https://github.com/CocoaPods/metrics.cocoapods.org/issues/18
Sister PR: https://github.com/CocoaPods/Humus/pull/6
Regarding the migration I was unsure if I should also add the migration in this repository, but if I understand it correctly the migrations in this repo have been deprecated?(shouldn't they be removed in that case)

Also I didn't not get trunk setup so I haven't actually ran the specs and I still need to add one for the language related changes I guess