remove Ruby 1.9.2 from Travis CI configuration

The "listen" gem is in the dependency chain for bogus, and this listen
gem has a hard requirement on Ruby 1.9.3 or later. This causes the bogus
travis builds to fail on Ruby 1.9.2.

Since Ruby 1.9.2 is deprecated, remove it from the Travis configuration.
Merge pull request #52 from ktdreyer/rm-ruby192-travis

remove Ruby 1.9.2 from Travis CI configuration