travis: update the system rubygems version

This works around a version incompatibility between bundler and rubygems.

Author: carlosmn

.travis.yml

@@ -34,6 +34,7 @@ git:
 
 before_install:
   - git submodule update --init
+  - gem update --system
   - if [ "$TRAVIS_OS_NAME" = "osx" ]; then ./vendor/libgit2/script/install-deps-osx.sh; fi
 
 script: script/travisbuild