Update bundler to last in 1.15.x

Author: JonRowe

.travis.yml

@@ -25,8 +25,8 @@ bundler_args: "--binstubs --path ../bundle --retry=3 --jobs=3"
 before_install:
   - script/clone_all_rspec_repos
   # Note this doesn't work on JRUBY 2.0.0 mode so we don't do it, the excluded versions are broken on Ruby 2.3
-  - if [ "jruby" != "$TRAVIS_RUBY_VERSION"  ]; then gem install bundler --version "1.11.2"; fi
-  - alias bundle="bundle _1.11.2_"
+  - if [ "jruby" != "$TRAVIS_RUBY_VERSION"  ]; then gem install bundler --version "1.15.4"; fi
+  - alias bundle="bundle _1.15.4_"
 
 before_script:
   # Rails 4.x complains with a bundler rails binstub in PROJECT_ROOT/bin/