(SERVER-2680) Update Travis test script to allow test filters
(SERVER-2680) Update Travis test script to allow test filters

This commit updates the travis test script to respect the
`MULTITHREADED` environment variable. The filter will default to
`:singlethreaded`, since that will be the default for the toggles
that configure running in multithreaded mode. When `MULTITHREADED=true`,
the `:multithreaded` test filter will be used.

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