Remove old columns from job and build

- Remove compatibility code for status and result from build
- Remove compatibility code for status from job
- Change code using symbols for state to use strings
- Adding migrations to change jobs and builds with state "finished"
- Remove unused columns from job and build
Don't denormalize the language key on builds

This was removed from the database, and is not being used.
Mark builds and jobs with NULL results as errored

Looked through the database and the only jobs/builds matching this are
jobs from before we added error states, but where the worker would error
and spew out an exception in the log.

For example: https://travis-ci.org/svenfuchs/globalize3/jobs/849524
Remove status fields from the factories
Remove builds.recent from find_builds

The behaviour was changed in 73782013c0585f6de5be159b33905b8ab0146e56,
and this is the only place where that scope is still used.
Build#older_than already handles the number being nil, so we can just
remove the entire conditional around that.
Merge pull request #212 from travis-ci/hh-remove-old-builds-and-jobs-columns

Remove old columns from job and build