Remove ruby 1.9.2 from our travis configuration.

Ruby 1.9.2 hasnt been supported anymore for quite a while: 

>> Effective immediately, 1.8.7 and 1.9.2 will be supported for security patches until June 2014.

from: https://www.ruby-lang.org/en/news/2013/12/17/maintenance-of-1-8-7-and-1-9-2/

This PR should also fix: https://github.com/troessner/reek/pull/303