Use #update_column and #update_attributes for RemoveLog

As pointed out in https://github.com/travis-ci/travis-api/pull/122#issuecomment-48041880
`log.save!` might not be saving attributes correctly.
We are speculatively using `#update_column` and `#update_attributes`
to see if that solves the problem
Use update_attributes in one transaction
Use Log#clear! before adding log removal data
Tweak RemoveLog logic

1. Use removed_by as the foreign key for removed_by column in logs
2. Use update_columns instead of update_attributes
Use update_attributes

Since update_columns is only for Rails 4+.
Ensure log parts are aggregated correctly
Merge pull request #381 from travis-ci/ha-bug-remove_log_really

RemoveLog service improvements