Add Ruby 2.3.1 Travis CI matrix

- remove 1.9.3 support
- force newest bundler for CI

.travis.yml

@@ -1,14 +1,16 @@
 language: ruby
 script: "bundle exec rspec"
 rvm:
-  - 1.9.3
   - 2.0.0
   - 2.1.5
   - 2.2.1
+  - 2.3.1
 gemfile:
   - gemfiles/active_record_3.gemfile
   - gemfiles/active_record_4.gemfile
   - gemfiles/mongoid_3.gemfile
   - gemfiles/mongoid_4.gemfile
 services:
   - mongodb
+before_install:
+  - gem install bundler -v 1.12.5