Be more selective when killing stray Sidekiqs

Avoid the background_jobs script killing every process with 'sidekiq' in
its argument string (e.g. 'rake gitlab:sidekiq start') by also catching
the number (2) in 'sidekiq 2.14'.
Merge branch 'careful_sidekiq_kill' into 'master'

Be more selective when killing stray Sidekiqs