Consolidate jenkins scripts

We had four separate jenkins scripts, only two of which were actually
being used: jenkins-new.sh (by govuk_whitehall_branches) and
jenkins-parallel.sh (by govuk_whitehall). This gets rid of all but one
(jenkins-new.sh, renamed to jenkins.sh) as this should work fine for
both build projects.
Merge pull request #1240 from alphagov/consolidate-jenkins-scripts

Consolidate jenkins scripts

We had four separate jenkins scripts, only two of which were actually
being used: jenkins-new.sh (by govuk_whitehall_branches) and
jenkins-parallel.sh (by govuk_whitehall). This gets rid of all but one
(jenkins-new.sh, renamed to jenkins.sh) as this should work fine for
both build projects.