Gemfile.lock: bump rack version
Gemfile.lock: bump rack version

This fixes CVE-2018-16471, an XSS vulnerability that almost certainly
doesn't impact us anyway.

mosoft521/gitscm-next,Mokolea/git-scm.com,git/git-scm.com,mosoft521/gitscm-next,mosoft521/gitscm-next,Mokolea/git-scm.com,git/git-scm.com,mosoft521/gitscm-next,git/git-scm.com,git/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com