add purged_at to logs table
add purged_at to logs table

as we remove the content of logs from the db we can keep track of the date we did that
Tiger66639/travis-core,travis-ci/travis-core,GistIcon/travis-core,final-ci/travis-core,kidaa/travis-core,jantman/travis-core,GistIcon/travis-core,GistIcon/travis-core,final-ci/travis-core,jantman/travis-core,Tiger66639/travis-core,Tiger66639/travis-core,travis-ci/travis-core,kidaa/travis-core,travis-ci/travis-core,developerfm/travis-core,developerfm/travis-core,developerfm/travis-core,kidaa/travis-core,final-ci/travis-core