Update bundler before travis builds. (rspec#1869)

Author: RustyNail

.travis.yml

@@ -22,7 +22,9 @@ cache: bundler
 
 bundler_args: "--binstubs --without documentation --path ../bundle --retry=3 --jobs=3"
 
-before_install: "script/clone_all_rspec_repos"
+before_install:
+  - script/clone_all_rspec_repos
+  - gem update bundler
 
 before_script:
   # Rails 4.x complains with a bundler rails binstub in PROJECT_ROOT/bin/