(SERVER-2100) Install lein-exec plugin when in CI
(SERVER-2100) Install lein-exec plugin when in CI

The lein-exec plugin is needed for the updated lein release job. This
commit ensures the plugin gets installed for use in Jenkins.

puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppet-server,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppetserver,puppetlabs/puppet-server