Remove Ruby 1.9.2 from Travis runs

This started failing on 1.9.2 and there's little incentive to fix it.

Change-Id: I50997be1fea36a51ac322dd714fc076558a4c33b
Reviewed-on: http://gerrit.causes.com/38448
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>