Update system gem in travis to avoid issue with rainbow gem

Ivo Anjo · Ivo Anjo · commit 1779fee9988b · 2017-02-17T21:50:35.000Z
See ku1ik/rainbow#48 for more details.
diff --git a/.travis.yml b/.travis.yml
@@ -19,4 +19,8 @@ matrix:
     - rvm: jruby-head
     - rvm: rbx-2
 
+before_install:
+  - gem update --system
+  - gem install bundler
+
 bundler_args: --without development
