Running Ruby linter for PdfHandler

See also commits in PdfHandler[1], mediawiki/selenium[2], jenkins-job-builder-config[3].

1: https://gerrit.wikimedia.org/r/#/c/143175/
2: https://gerrit.wikimedia.org/r/#/c/144984/
3: https://gerrit.wikimedia.org/r/#/c/152246/

Change-Id: I1b2bcc71352abbd67665b22634a8ff78e0d68ffc