(maint) Make test gem install respect LEIN env var
(maint) Make test gem install respect LEIN env var

Jenkins uses the `LEIN` env var to control the version of lein to use in
its jobs. This commit updates our script to install gems for testing to
respect this var.

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