Always use newest bundler in the CI

amatsuda · amatsuda · commit 58473062db8c · 2017-09-07T06:30:10.000+09:00
This fixes "NoMethodError: undefined method `spec' for nil:NilClass" on 1.9.3
and similar errors in the future
diff --git a/.travis.yml b/.travis.yml
@@ -2,6 +2,8 @@ language: ruby
 cache: bundler
 sudo: false
 
+before_install: gem update bundler
+
 rvm:
   - 1.9.3
   - 2.0.0
