Pass comma separated list of files to brakeman

When a space separated list of files is passed to brakeman with the
`--only-files` option, brakeman fails with the error 'Please supply the
path to a Rails application.' When the files are passed as a comma
separated string, all is well.

Change-Id: I52f998b395d9393b646b1896817e3905c12dfbec
Reviewed-on: http://gerrit.causes.com/43692
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>