update travis config, test on ruby 2.2.5 and 2.3.1 only, update rubygems and bundler before running tests

Author: manuelmeurer

.travis.yml

@@ -1,6 +1,7 @@
 language: ruby
 rvm:
- - 1.9
- - 2.0
- - 2.1
- - jruby
+ - 2.2.5
+ - 2.3.1
+before_install:
+ - gem update --system
+ - gem update bundler