Use JENKINS_URL as environment variable

Summary:
As Hudson isn't maintained anymore, it's more make sense to use
the default product name instead of the legacy one as variable.

Ref T1448.

Test Plan:

$ setenv JENKINS_URL https://ci.nasqueron.org
$ bundle exec rackup

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1448

Differential Revision: https://devcentral.nasqueron.org/D1792