Update .travis.yml
Update .travis.yml

Drop ruby 1.9.3 - support ended on 2016-02-23. 
See: https://www.ruby-lang.org/en/news/2014/01/10/ruby-1-9-3-will-end-on-2015/
excpt/ruby-jwt,ab320012/ruby-jwt,jwt/ruby-jwt,progrium/ruby-jwt