test against ruby 2.4 (#74)
test against ruby 2.4 (#74)

Updates the travis CI configuration to unpin all of the tested ruby versions, and adds 2.4 to the versions list to test against. Eventually, we may want to consider removing ruby 2.1 from the tested version set as it will no longer be maintained in April 2018. Perhaps when 2.1 is removed we can add 2.5 since it should have a non-preview release by then.
senny/sablon,fullfabric/sablon